A dominant7th on 4 shouldn't exist in the key — its b7 is the home key's b3, the blue note itself. Play 4⁷ where plain 4 belongs and the whole progression puts on sunglasses. This is bar five of every 12-bar, extracted and reusable anywhere.
hear it in bar five of every 12-bar blues ever played
Lead-in — how players set it upFrom plain 4 — sour it to 4⁷ mid-bar for the blues turn.Or off 1⁷ — the whole neighborhood flat-sevens together.Or from 5⁷, cycling dominants shamelessly.Follow — where it usually goesBack to 1⁷ — in blues even home wears the b7.The #4 dim7 splits 4⁷ from 1 — the gospel bridge.Or ride 4⁷ up: 5⁷ answers and the 12-bar turns.
The 4⁷'s b7 is the home key's b3 — the blue note, now legal. Bend into it; that's the whole genre in one pitch.Target tones (in C)C7 — lean on E (3) · Bb (b7)F7 — lean on A (3) · Eb (b7)C7 — lean on E (3) · Bb (b7)Half-step wiresE → F · Bb ↘ A · F ↘ E · A → Bb