Blues scale
Minor pentatonic with an added diminished fifth, the blue note.
What it means
The blues scale consists of the minor pentatonic scale — root, minor third, fourth, fifth, minor seventh — and an additional note between the fourth and the fifth, written as a diminished fifth. This added note is called the blue note. In all, this yields six notes.
Musical use
The distinctive feature of this scale is that it goes together with an accompaniment made up predominantly of major chords. The minor third of the melody meets the major third of the harmony — a friction that is felt not as a mistake but as the core of the sound. The same is true of the blue note: it does not demand resolution like an ordinary dissonance, but stays put as a colouring or is passed through.
Notation, moreover, only approximates what actually sounds. In lived practice, the notes in question often do not lie exactly on the tempered degrees but in between, and are shaped by pulling, sliding, and bending the pitch. The scale is therefore less a fixed scale than a description of a pool of notes whose execution is flexible.
It is to be distinguished from the minor pentatonic scale, to which it adds only one note, and from the Mixolydian scale, which also plays a role in the same music but contains a major third.
The blue notes can only be represented approximately in the tempered system. Sung and played on guitar or wind instruments, the lowered third, fifth, and seventh often lie between the notes of the piano, and the notes are pulled or slurred. The blues scale is therefore more a pool of characteristic notes than a fixed scale. Over the major chords of the twelve-bar blues, a deliberate friction arises between the minor third of the melody and the major third of the accompaniment.
Performance practice
Do not treat the blue note as a passing note that you leave quickly, but as a colour that needs time. On instruments with flexible pitch, work with pulling and sliding rather than exact semitones; on the piano, it helps to place neighbouring notes close together in quick succession. Listen to recordings and imitate them — this music lives from a practice that can be notated only to a limited extent.