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.
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.
