A major feature of the blues scale is the use of blue notes; however, since blue notes are considered alternative inflections, a blues scale may be considered to not fit the traditional definition of a scale. At its most basic, a single version of this blues scale is commonly used over all changes in a twelve bar blues progression. Likewise, in contemporary jazz theory, its use is commonly based upon the key rather than the individual chord. The evolution of this scale may be traced back to Asia through native North America with the addition of the flat-5 blue note. Greenblatt defines two blues scales, the major and the minor. The major blues scale is 1, 2,3, 3, 5, 6 and the minor is 1, 3, 4, 5, 5, 7. The latter is the same as the hexatonic scale described above.
Heptatonic
The heptatonic, or seven-note, conception of the blues scale is as a diatonic scale with lowered third, fifth, and seventh degrees, which is equivalent to the dorian 5 scale, the second mode of the harmonic major scale. Blues practice is derived from the "conjunction of 'African scales' and the diatonic western scales".
Steven Smith argues that, "to assign blue notes to a 'blues scale' is a momentous mistake, then, after all, unless we alter the meaning of 'scale'".
Nonatonic
An essentially nine-note blues scale is defined by Benward and Saker as a chromatic variation of the major scale featuring a flat third and seventh degrees which, "alternating with the normal third and seventh scale degrees are used to create the blues inflection. These 'blue notes' represent the influence of African scales on this music."
A different and non-formal way of playing the scale is by the use of quarter tones, added to the 3rd and 7th degrees of the minor blues scale. For example, the A minor blues scale with quarter tones is A–B–C–D–E–F–G, where is a half sharp. Also, the note D can be used as an additional note. Guitar players can raise a given note by a quarter tone through bending.
Usage
In jazz, the blues scale is used by improvising musicians in a variety of harmonic contexts. It can be played for the entire duration of a twelve bar blues progression constructed off the root of the first dominant seventh chord. For example, a C hexatonic blues scale could be used to improvise a solo over a C blues chord progression. The blues scale can also be used to improvise over a minor chord. Jazz educator Jamey Aebersold describes the sound and feel of the blues scale as "funky," "down-home," "earthy," or "bluesy." The blues scale is also used in other genres to reference the blues idiom.
