The blues isn’t really compatible with conventional tonality: it’s basically major chords with added minor seventh (I won’t call them dominant sevenths because they don’t function as dominant chords), with minor pentatonic (plus added flat fifth) melody. There’s no way that combination can possibly work—but it does!
IMHO that’s a narrow definition of the Blues, and the genre is much wider. From the top of my head, I’m thinking of BB King playing “The Thrill is Gone”. Still blues, but in a minor key. Definitely not major chords with an added minor seventh.
For the minor blues scale working over a major blues progression - I think the dissonance is okay because the flat third and fifth are often passing tones. If you loiter on them, they are more jarring.
I wouldn't say the minor third is a "passing tone" at all, it's the defining note of the minor scale. Listen to e.g. "Mannish Boy"--the minor third is absolutely essential to that riff.
Agree that of course there are blues songs with definitely minor harmony, but I still think the really distinctive and innovative characteristic of blues is "minor melody over major harmony"--the minor pentatonic scale was already ubiquitous in African-American music in e.g. spirituals. That's just incredibly weird and I can't imagine how it must have sounded to white audiences hearing it for the first time.
I think you’re misunderstanding the context I was replying to.
Of course a minor third is important to a minor scale. But if the underlying harmony happens to be a major chord, then the minor third is dissonant relative to the harmony.
I referred to this dissonance as a passing tone because you said “ There’s no way that combination can possibly work—but it does!”.
Yeah, the added flat fifth really isn't a Western European thing, that's what made it different, cool and appealing. Also the shuffle rhythm.
But if you listen to a lot of blues, play a lot of it, play with other blues players, etc. You will notice there's a vocabulary, idioms, etc. You can learn them by ear. You can call them by names of songs or players (Bo Diddley beat), or by the number of bars, ... Well, all of that is kind of - theory. Also, knowing which things you wouldn't play because they don't fit the style, that's also theory.
