Singing a flatted third over an accompanying major I, for instance, was common among black and white folk musicians in the South, and I've heard examples of it from Europe and Africa. Suppose you think similar to 1b345b7 is a "normal" scale, among a lot of people you know, and a lot of those people are learning major chords as they learn to play instruments, and are combining a scale similar to that with accompaniment with major chords. Then that sounds normal to you.

Handy was proud that some people thought it was interesting when he threw flatted thirds into his scores in places an Abbe Niles wouldn't expect. But that was because Niles lived in Connecticut. When Richard M. Jones (born near Baton Rouge, LA) wrote about a tune with the same chord progression as "Bucket's Got A Hole In It" as "the oldest blues in the world," he was speaking from personal experience that Abbe Niles didn't have.

Blues notes only sounded like wrong notes to _some_ people.