For Blues players, the Pentatonic scales are where it all begins.
The Pentatonic Scale is a 5 note scale (”penta” means 5). The most common Pentatonic scale is the Minor Pentatonic scale.
The minor pentatonic scales consists of the 1, b3, 4, 5, and b7 scale notes.
In every major key, 3 minor pentatonic scales occur diatonically. They are built off the 2nd, 3rd, and 6th degrees of the scale.
Over a major chord the minor pentatonic on the 6th degree.
Over the dominant and minor-type chords, the minor pentatonic built on the root is often used.
Over minor types chord you can also build in the b3rd.
Dominant-type chords contain a major 3rd, so the pentatonic’s b3 creates a little bit of a rub. But this clash is actually desirable in blues music and adds a distinctly bluesy quality. Sometimes, when the b3 is played over a dominant-type chords, blues players like to bend up to the major 3rd. Sometimes they deliberately explloit the tension of sounding the notes “between the cracks”.
Technically, if you are playing a b3 over a dominant chord, you are playing a #9. The #9 is very familiar in blues. Like the E7#9 chord.