Originally posted by chris mood
all of music theory is based off the major scale system that was created by the Greeks thousands of years ago. The music and scales of the Blues, wich is mainly responsible for all of contemporary music,is only a couple of hundred yrs. old. People are always looking for some way of relating the 2 systems together (diatonic & Blues)but in reality they are very different from each other and should be approached so.
The harmonic and melodic approach to the blues is very different then the traditional diatonic system. The blues is based off of minor scales being applied over top of major or dominant 7th harmonies. The 1,4,5 progression is always referenced to the traditional diatonic harmony (1maj.7 4maj7 5dom7) unless otherwise specified (1dom7 4dom7 5dom7)
so to answer your question, the 1 4 5 prog. does not refer to all dom.7th chords unless the author is specifically refering to a Blues prog.

I understand your point regarding the application of the I, IV, V as it applies to diatonic harmony as opposed to blues. And maybe I should have been a little clearer in my use of terminology. I'm just wondering if all of the notes which make up the chords of the I7, IV7, V7 progression are contained in one scale, similar to the way that all of the notes which make up the chords of the I, IV, V are contained in the Major Scale.