The blues scale is a six note scale. The scale can be constructed by starting with a major scale and then using the following formula:
The C blues scale would be C-Eb-F-F#-G-Bb. This scale works very nicely over minor 7th chords.
A major blues scale also exists and can be constructed from a major scale using the following formula:
The major blues scale works on major and dominant chords. On the surface it looks like you have to learn 12 minor blues scales and then 12 major blues scales but dont fret.
Notice that a D minor blues scale consists of D-F-G-G#-A-C. Notice that an F major blues scale consists of F-G-Ab-A-C-D.
If you look closely you should note the the notes in a D minor blues scale are identical to an F major blues scale.
The practical outcome is that you only need to learn the 12 minor blues scales because you can play the minor blues scale that is down a minor third from a major or dominant 7 chord.
For example a simple 12 bar blues in the key of C would be:
You can play the A minor blues scale over the C7.
You can play the D minor blues scale over the F7.
You can play the E minor blues scale over the G7.
Have fun playing the blues!