Watch the Dominant Seven online guitar lesson by Robben Ford from Chord Revolution: Foundations
I'm going to play a blues for you now in the key of A. So the first chord is going to be A7. Now, what would the scale be in the key of A? Any seventh chord, dominant seventh chord, is a five chord. So that means A7 is going to be the five chord in the key of D. So D major is your scale. You're just starting on the fifth degree, or the five chord in the key of D. So each of the three chords in a blues are seventh chords, therefore, they're five chords, theoretically speaking and each one has its own scale. You're changing keys when you go to the four chord of a blues. Your scale there is G, G major. Likewise, when you go to the E7 chord your scale is going to be A major. So you are changing keys every time you go to one of these chords in the blues. Good thing to know.