7th extensions are different from 7th chords, which are meant as dominant 7 chords. Every chord can have a 7th note added as an extension to its chord structure, either a minor or major depending on what chord it is. If it is the I chord, it is a major 7th - ie, a B in a Cmaj7 chord. on the II and III chords it is a minor 7th. On the IV chord it is a major 7th. On the V chord it is a minor 7th, which makes the chord a dominant 7th chord, as it is the only chord based off the major scale that has a major third and a minor 7th. On the VI and VII chord it is a minor 7. Now, If you are playing a chord like E7, it implies that the E is the V of a chord. If it has a major7th in it is is written like Cmaj7 or CM7, if it is a minor chord it is written Cmin7, or Cm7. In blues, however, you play a I, IV, V progression and all of them are dominant 7 chords. Strange, yes, but blues doesn't fit easily into common theory. Part of what makes the blues sound like blues is the way in which a minor pentatonic scale works over the dominant chords of I, IV, V. In other words, after all of this, you are playing the right thing for blues. If you were to play a scale over a V chord in jazz, you would generally play a mixolydian scale of the same note, ie G mixolydian over G7. That would mean the you could play A mixolydian, D mixolydian and E mixolydian over a I, IV, V blues progression in A, but it wouldn't sound like blues. Give it a go though, it can sound kind of nice.