Blues shows that traditional theory can't explain everything. There are a couple of approaches. The blues scale can be used from the relative minor of the key, so that in the key of C, with no sharps, you will have an Amin blues scale. This is more of a rock thing. It is more common in blues to play a C blues in the key of C, with the chords being C7 F7, G7. Of course, if this were written out there would be a hell of a lot of accidentals, becuase the C blues scale has a minor third, a flat 5th, and a dominant 7. I think I might not be helping very much so i will stop in the hope that someone can more coherantly tell you.