Playing The Blues

I may have mentioned before here... that last year I picked up the guitar again after many years and set myself to learning to play the blues. I must admit that I haven't been as diligent as I should have been but I am making progress.

I was thinking today as I was trying to understand some 'theory' about the early blues men that bubbled out of the Mississippi Delta. Did these guys worry or even know anything about musical theory? Did they think about flatted fifth pentatonic scales and 7th chords versus major or did they just play from the heart? Did it just happen by accident that blues relies on the flatted fifth scale and the 7th chord because those sound the 'bluest' or did someone do it all on purpose?

I'm not sure this is really important but maybe one of you musicians out there knows more about the history of the blues than I. How much musical 'theory' do you think Robert Johnson really understood?