Originally Posted by MalcolmAmos
Point - do not know why he went to the flat chords.
I think the answer to "why" has to do with how we think of a Blues chord progression...or, more specifically, how most musicians don't
think of it!
It's all well & good to think of the harmonic functions of the chord changes in precise musical terms, but it's also really important to think more abstractly about how non-musician listeners hear
those harmonic functions:
- Your tonic chord(s) sound stable, comforting, like home. They're "this thing"
- Your subdominant chord(s) sound a bit less stable, less comforting...they're definitely "this other
thing" that's a bit different from home
- Your dominant chord(s) and or all substitutions sound as far away from home as you can possibly get; they are very conspicuously "not that first thing at all!"
So in this abstract layperson perception, a Blues progression simply goes Some Of This, Some Of This Other
Thing, Some More Of This Again, and then This Whole Different Thing Entirely.
context, Bb and Eb major triads in the key of C definitely fullfill the Whole Different Thing Entirely criteria.