diff --git a/packages/tailwindcss-language-server/src/projects.ts b/packages/tailwindcss-language-server/src/projects.ts
index 25aacd5f0..04160569e 100644
--- a/packages/tailwindcss-language-server/src/projects.ts
+++ b/packages/tailwindcss-language-server/src/projects.ts
@@ -837,6 +837,7 @@ export async function createProjectService(
         )
 
         state.designSystem = designSystem
+        state.blocklist = Array.from(designSystem.invalidCandidates ?? [])
 
         let deps = designSystem.dependencies()
 
@@ -982,7 +983,9 @@ export async function createProjectService(
       if (typeof state.separator !== 'string') {
         state.separator = ':'
       }
-      state.blocklist = Array.isArray(state.config.blocklist) ? state.config.blocklist : []
+      if (!state.v4) {
+        state.blocklist = Array.isArray(state.config.blocklist) ? state.config.blocklist : []
+      }
       delete state.config.blocklist
 
       if (state.v4) {
@@ -1148,6 +1151,7 @@ export async function createProjectService(
       state.designSystem = designSystem
       state.classList = classList
       state.variants = getVariants(state)
+      state.blocklist = Array.from(designSystem.invalidCandidates ?? [])
 
       let deps = designSystem.dependencies()
 
diff --git a/packages/tailwindcss-language-server/tests/diagnostics/diagnostics.test.js b/packages/tailwindcss-language-server/tests/diagnostics/diagnostics.test.js
index 56fe9f7ff..afda88373 100644
--- a/packages/tailwindcss-language-server/tests/diagnostics/diagnostics.test.js
+++ b/packages/tailwindcss-language-server/tests/diagnostics/diagnostics.test.js
@@ -1,6 +1,8 @@
+import * as fs from 'node:fs/promises'
 import { expect, test } from 'vitest'
 import { withFixture } from '../common'
-import * as fs from 'node:fs/promises'
+import { css, defineTest } from '../../src/testing'
+import { createClient } from '../utils/client'
 
 withFixture('basic', (c) => {
   function testFixture(fixture) {
@@ -383,3 +385,43 @@ withFixture('v4/basic', (c) => {
     ],
   })
 })
+
+defineTest({
+  name: 'Shows warning when using blocklisted classes',
+  fs: {
+    'app.css': css`
+      @import 'tailwindcss';
+      @source not inline("{,hover:}flex");
+    `,
+  },
+  prepare: async ({ root }) => ({ client: await createClient({ root }) }),
+  handle: async ({ client }) => {
+    let doc = await client.open({
+      lang: 'html',
+      text: '<div class="flex underline hover:flex">',
+    })
+
+    let diagnostics = await doc.diagnostics()
+
+    expect(diagnostics).toEqual([
+      {
+        code: 'usedBlocklistedClass',
+        message: 'The class "flex" will not be generated as it has been blocklisted',
+        range: {
+          start: { line: 0, character: 12 },
+          end: { line: 0, character: 16 },
+        },
+        severity: 2,
+      },
+      {
+        code: 'usedBlocklistedClass',
+        message: 'The class "hover:flex" will not be generated as it has been blocklisted',
+        range: {
+          start: { line: 0, character: 27 },
+          end: { line: 0, character: 37 },
+        },
+        severity: 2,
+      },
+    ])
+  },
+})
diff --git a/packages/tailwindcss-language-service/src/diagnostics/diagnosticsProvider.ts b/packages/tailwindcss-language-service/src/diagnostics/diagnosticsProvider.ts
index 18994526c..075cea38e 100644
--- a/packages/tailwindcss-language-service/src/diagnostics/diagnosticsProvider.ts
+++ b/packages/tailwindcss-language-service/src/diagnostics/diagnosticsProvider.ts
@@ -9,6 +9,7 @@ import { getInvalidConfigPathDiagnostics } from './getInvalidConfigPathDiagnosti
 import { getInvalidTailwindDirectiveDiagnostics } from './getInvalidTailwindDirectiveDiagnostics'
 import { getRecommendedVariantOrderDiagnostics } from './getRecommendedVariantOrderDiagnostics'
 import { getInvalidSourceDiagnostics } from './getInvalidSourceDiagnostics'
+import { getUsedBlocklistedClassDiagnostics } from './getUsedBlocklistedClassDiagnostics'
 
 export async function doValidate(
   state: State,
@@ -22,6 +23,7 @@ export async function doValidate(
     DiagnosticKind.InvalidTailwindDirective,
     DiagnosticKind.InvalidSourceDirective,
     DiagnosticKind.RecommendedVariantOrder,
+    DiagnosticKind.UsedBlocklistedClass,
   ],
 ): Promise<AugmentedDiagnostic[]> {
   const settings = await state.editor.getConfiguration(document.uri)
@@ -52,6 +54,9 @@ export async function doValidate(
         ...(only.includes(DiagnosticKind.RecommendedVariantOrder)
           ? await getRecommendedVariantOrderDiagnostics(state, document, settings)
           : []),
+        ...(only.includes(DiagnosticKind.UsedBlocklistedClass)
+          ? await getUsedBlocklistedClassDiagnostics(state, document, settings)
+          : []),
       ]
     : []
 }
diff --git a/packages/tailwindcss-language-service/src/diagnostics/getUsedBlocklistedClassDiagnostics.ts b/packages/tailwindcss-language-service/src/diagnostics/getUsedBlocklistedClassDiagnostics.ts
new file mode 100644
index 000000000..07f1084c5
--- /dev/null
+++ b/packages/tailwindcss-language-service/src/diagnostics/getUsedBlocklistedClassDiagnostics.ts
@@ -0,0 +1,41 @@
+import type { TextDocument } from 'vscode-languageserver-textdocument'
+import type { State, Settings } from '../util/state'
+import { type UsedBlocklistedClassDiagnostic, DiagnosticKind } from './types'
+import { findClassListsInDocument, getClassNamesInClassList } from '../util/find'
+
+export async function getUsedBlocklistedClassDiagnostics(
+  state: State,
+  document: TextDocument,
+  settings: Settings,
+): Promise<UsedBlocklistedClassDiagnostic[]> {
+  if (!state.v4) return []
+  if (!state.blocklist?.length) return []
+
+  let severity = settings.tailwindCSS.lint.usedBlocklistedClass
+  if (severity === 'ignore') return []
+
+  let blocklist = new Set(state.blocklist ?? [])
+  let diagnostics: UsedBlocklistedClassDiagnostic[] = []
+
+  let classLists = await findClassListsInDocument(state, document)
+
+  for (let classList of classLists) {
+    let classNames = getClassNamesInClassList(classList, [])
+
+    for (let className of classNames) {
+      if (!blocklist.has(className.className)) continue
+
+      diagnostics.push({
+        code: DiagnosticKind.UsedBlocklistedClass,
+        range: className.range,
+        severity:
+          severity === 'error'
+            ? 1 /* DiagnosticSeverity.Error */
+            : 2 /* DiagnosticSeverity.Warning */,
+        message: `The class "${className.className}" will not be generated as it has been blocklisted`,
+      })
+    }
+  }
+
+  return diagnostics
+}
diff --git a/packages/tailwindcss-language-service/src/diagnostics/types.ts b/packages/tailwindcss-language-service/src/diagnostics/types.ts
index 7cb68a7ed..f022503aa 100644
--- a/packages/tailwindcss-language-service/src/diagnostics/types.ts
+++ b/packages/tailwindcss-language-service/src/diagnostics/types.ts
@@ -10,6 +10,7 @@ export enum DiagnosticKind {
   InvalidTailwindDirective = 'invalidTailwindDirective',
   InvalidSourceDirective = 'invalidSourceDirective',
   RecommendedVariantOrder = 'recommendedVariantOrder',
+  UsedBlocklistedClass = 'usedBlocklistedClass',
 }
 
 export type CssConflictDiagnostic = Diagnostic & {
@@ -100,6 +101,16 @@ export function isRecommendedVariantOrderDiagnostic(
   return diagnostic.code === DiagnosticKind.RecommendedVariantOrder
 }
 
+export type UsedBlocklistedClassDiagnostic = Diagnostic & {
+  code: DiagnosticKind.UsedBlocklistedClass
+}
+
+export function isUsedBlocklistedClass(
+  diagnostic: AugmentedDiagnostic,
+): diagnostic is UsedBlocklistedClassDiagnostic {
+  return diagnostic.code === DiagnosticKind.UsedBlocklistedClass
+}
+
 export type AugmentedDiagnostic =
   | CssConflictDiagnostic
   | InvalidApplyDiagnostic
@@ -109,3 +120,4 @@ export type AugmentedDiagnostic =
   | InvalidTailwindDirectiveDiagnostic
   | InvalidSourceDirectiveDiagnostic
   | RecommendedVariantOrderDiagnostic
+  | UsedBlocklistedClassDiagnostic
diff --git a/packages/tailwindcss-language-service/src/util/state.ts b/packages/tailwindcss-language-service/src/util/state.ts
index 8c819eb8f..754f2a497 100644
--- a/packages/tailwindcss-language-service/src/util/state.ts
+++ b/packages/tailwindcss-language-service/src/util/state.ts
@@ -65,6 +65,7 @@ export type TailwindCssSettings = {
     invalidTailwindDirective: DiagnosticSeveritySetting
     invalidSourceDirective: DiagnosticSeveritySetting
     recommendedVariantOrder: DiagnosticSeveritySetting
+    usedBlocklistedClass: DiagnosticSeveritySetting
   }
   experimental: {
     classRegex: string[] | [string, string][]
@@ -203,6 +204,7 @@ export function getDefaultTailwindSettings(): Settings {
         invalidTailwindDirective: 'error',
         invalidSourceDirective: 'error',
         recommendedVariantOrder: 'warning',
+        usedBlocklistedClass: 'warning',
       },
       showPixelEquivalents: true,
       includeLanguages: {},
diff --git a/packages/tailwindcss-language-service/src/util/v4/design-system.ts b/packages/tailwindcss-language-service/src/util/v4/design-system.ts
index 20b7ff323..13c657c33 100644
--- a/packages/tailwindcss-language-service/src/util/v4/design-system.ts
+++ b/packages/tailwindcss-language-service/src/util/v4/design-system.ts
@@ -40,6 +40,7 @@ export interface DesignSystem {
 
   // Optional because it did not exist in earlier v4 alpha versions
   resolveThemeValue?(path: string, forceInline?: boolean): string | undefined
+  invalidCandidates?: Set<string>
 }
 
 export interface DesignSystem {
diff --git a/packages/vscode-tailwindcss/CHANGELOG.md b/packages/vscode-tailwindcss/CHANGELOG.md
index 17f07afc2..4c9240f60 100644
--- a/packages/vscode-tailwindcss/CHANGELOG.md
+++ b/packages/vscode-tailwindcss/CHANGELOG.md
@@ -2,7 +2,7 @@
 
 ## Prerelease
 
-- Nothing yet!
+- Warn when using a blocklisted class in v4 ([#1310](https://github.com/tailwindlabs/tailwindcss-intellisense/pull/1310))
 
 # 0.14.15
 
diff --git a/packages/vscode-tailwindcss/README.md b/packages/vscode-tailwindcss/README.md
index efc937cc4..f74b14072 100644
--- a/packages/vscode-tailwindcss/README.md
+++ b/packages/vscode-tailwindcss/README.md
@@ -172,6 +172,10 @@ Class names on the same HTML element which apply the same CSS property or proper
 
 Class variants not in the recommended order (applies in [JIT mode](https://tailwindcss.com/docs/just-in-time-mode) only). **Default: `warning`**
 
+#### `tailwindCSS.lint.usedBlocklistedClass`
+
+Usage of class names that have been blocklisted via `@source not inline(…)`. **Default: `warning`**
+
 ### `tailwindCSS.inspectPort`
 
 Enable the Node.js inspector agent for the language server and listen on the specified port. **Default: `null`**
diff --git a/packages/vscode-tailwindcss/package.json b/packages/vscode-tailwindcss/package.json
index 26952a45e..d3d30911e 100644
--- a/packages/vscode-tailwindcss/package.json
+++ b/packages/vscode-tailwindcss/package.json
@@ -305,6 +305,17 @@
           "markdownDescription": "Class variants not in the recommended order (applies in [JIT mode](https://tailwindcss.com/docs/just-in-time-mode) only)",
           "scope": "language-overridable"
         },
+        "tailwindCSS.lint.usedBlocklistedClass": {
+          "type": "string",
+          "enum": [
+            "ignore",
+            "warning",
+            "error"
+          ],
+          "default": "warning",
+          "markdownDescription": "Usage of class names that have been blocklisted via `@source not inline(…)`",
+          "scope": "language-overridable"
+        },
         "tailwindCSS.experimental.classRegex": {
           "type": "array",
           "scope": "language-overridable"