diff --git a/frontend/src/canvas/markdownStyles.ts b/frontend/src/canvas/markdownStyles.ts
new file mode 100644
index 0000000..259c864
--- /dev/null
+++ b/frontend/src/canvas/markdownStyles.ts
@@ -0,0 +1,16 @@
+// Shared style constants + BlockNote dark-theme CSS string for injection
+
+export const MD_BLOCKNOTE_DARK_CSS = `
+  .bn-container[data-theme="dark"] {
+    --bn-colors-editor-background: #232336;
+    --bn-colors-editor-text: #e0e0e0;
+    --bn-colors-menu-background: #2a2a42;
+    --bn-colors-menu-text: #e0e0e0;
+    --bn-colors-tooltip-background: #333;
+    --bn-colors-tooltip-text: #ccc;
+    --bn-colors-hovered-background: rgba(121,80,242,0.15);
+    --bn-colors-selected-background: rgba(121,80,242,0.25);
+    --bn-colors-side-menu: #666;
+    --bn-font-family: Inter, system-ui, sans-serif;
+  }
+`;
diff --git a/frontend/src/components/MarkdownEditView.tsx b/frontend/src/components/MarkdownEditView.tsx
new file mode 100644
index 0000000..f505d07
--- /dev/null
+++ b/frontend/src/components/MarkdownEditView.tsx
@@ -0,0 +1,132 @@
+/**
+ * MarkdownEditView — BlockNote editor wrapper for markdown card edit mode.
+ * Lazy-loaded on first double-click via dynamic import().
+ */
+
+import React, { useCallback, useEffect, useRef } from 'react';
+import { useCreateBlockNote } from '@blocknote/react';
+import { BlockNoteViewRaw } from '@blocknote/react';
+import '@blocknote/react/style.css';
+
+interface MarkdownEditViewProps {
+  initialContent: string;
+  accentColor: string;
+  onSave: (newContent: string) => void;
+  onCancel: () => void;
+}
+
+// Markdown conversion is built into the BlockNote editor instance:
+// - editor.tryParseMarkdownToBlocks(md) — markdown → blocks
+// - editor.blocksToMarkdownLossy(blocks) — blocks → markdown
+// No separate @blocknote/xl-markdown package needed.
+
+export default function MarkdownEditView(props: MarkdownEditViewProps) {
+  const { initialContent, accentColor, onSave, onCancel } = props;
+  const savedRef = useRef(false);
+
+  const editor = useCreateBlockNote({
+    domAttributes: {
+      editor: {
+        'data-theme': 'dark',
+      },
+    },
+  });
+
+  // Initialize editor content from markdown
+  useEffect(() => {
+    const blocks = editor.tryParseMarkdownToBlocks(initialContent);
+    editor.replaceBlocks(editor.document, blocks);
+  }, []); // Only on mount
+
+  const doSave = useCallback(() => {
+    if (savedRef.current) return;
+    savedRef.current = true;
+    const md = editor.blocksToMarkdownLossy(editor.document);
+    onSave(md);
+  }, [editor, onSave]);
+
+  const doCancel = useCallback(() => {
+    if (savedRef.current) return;
+    savedRef.current = true;
+    onCancel();
+  }, [onCancel]);
+
+  // Ctrl+Enter to save, Escape to cancel
+  useEffect(() => {
+    const onKeyDown = (e: KeyboardEvent) => {
+      if (e.key === 'Enter' && (e.ctrlKey || e.metaKey)) {
+        e.preventDefault();
+        doSave();
+      } else if (e.key === 'Escape') {
+        e.preventDefault();
+        doCancel();
+      }
+    };
+    document.addEventListener('keydown', onKeyDown, true); // capture phase
+    return () => document.removeEventListener('keydown', onKeyDown, true);
+  }, [doSave, doCancel]);
+
+  // Click outside to save
+  useEffect(() => {
+    const onClick = (e: MouseEvent) => {
+      const target = e.target as HTMLElement;
+      if (!target.closest('[data-markdown-edit]')) {
+        doSave();
+      }
+    };
+    const timer = setTimeout(() => {
+      document.addEventListener('pointerdown', onClick);
+    }, 200);
+    return () => {
+      clearTimeout(timer);
+      document.removeEventListener('pointerdown', onClick);
+    };
+  }, [doSave]);
+
+  // Header with Done/Cancel controls
+  const headerStyle: React.CSSProperties = {
+    padding: '7px 14px',
+    background: '#2a2a42',
+    borderBottom: '1px solid #333',
+    display: 'flex',
+    alignItems: 'center',
+    justifyContent: 'space-between',
+    fontSize: '11px',
+  };
+
+  return (
+    <div
+      data-markdown-edit
+      style={{
+        border: `1.5px solid ${accentColor}`,
+        borderRadius: '10px',
+        overflow: 'hidden',
+        boxShadow: `0 4px 24px ${accentColor}33`,
+      }}
+    >
+      <div style={headerStyle}>
+        <span style={{ color: '#ccc' }}>Editing</span>
+        <div style={{ display: 'flex', gap: '8px', alignItems: 'center' }}>
+          <span style={{ color: '#666', fontSize: '10px' }}>Esc cancel</span>
+          <button
+            onClick={doSave}
+            style={{
+              background: accentColor,
+              color: '#fff',
+              border: 'none',
+              borderRadius: '4px',
+              padding: '3px 10px',
+              fontSize: '10px',
+              cursor: 'pointer',
+            }}
+          >
+            Done
+          </button>
+        </div>
+      </div>
+      <div style={{ minHeight: '100px' }}>
+        <BlockNoteViewRaw editor={editor} theme="dark" />
+      </div>
+    </div>
+  );
+}