The minor blues scale is the minor pentatonic plus one note, the b5 or "blue note" (1, b3, 4, b5, 5, b7). That extra half step adds grit and tension, the bluesy, crying sound behind countless blues and rock solos.
Formula
The reference key is A minor blues, the key all five diagrams below use.
| Degree | 1 | b3 | 4 | b5 | 5 | b7 |
|---|---|---|---|---|---|---|
| Note in A | A | C | D | Eb | E | G |
Intervals: 1½ W H H 1½ W (a step and a half, whole, half, half, a step and a half, whole). Remove the Eb and you're back to A minor pentatonic.
Sound and character
The b5 is a tritone above the root, the most unstable interval there is, so it never sounds like a place to rest. Played on its own it sounds wrong; squeezed between the 4 and the 5 it sounds like the blues. The chromatic run 4, b5, 5 (D, Eb, E) is the scale's signature.
Where it's used
Blues, blues-rock, hard rock, metal riffs, jazz blues, funk and soul. It fits minor chords (m7, m9), dominant 7th chords in a blues (A7, D7, E7 over a 12-bar in A), and power-chord riffs. In A, it sounds at home over Am7, Am to Dm, A7 vamps and any blues in A. Over a minor blues the scale fits all three chords (Am7, Dm7, Em7) with the same box.
How to move it
The root is the amber dot. To change key, slide every box so its roots land on the new key's note. Down 5 frets gives E minor blues (Box 1 at the open position, with the b5 on the A string, 1st fret); up 3 frets gives C minor blues (Box 1 at the 8th fret).
Practice: play the upper octave of Box 1 and say "b5" every time you pass the Eb, then play the dyads against the open A to hear the tritone rub and resolve.
The major blues scale is its bright twin: the major pentatonic plus the b3.