Another contributing factor to "blue" notes may have been the discordant sound of some traditional open banjo chords. While not strictly speaking "blue notes," that kind of "lonesome" banjo frailing would undoubtedly have accustomed people - white and black - to discords as a normal component of music.

Of course, maybe they developed the tunings because they already liked discords.

I've been looking for pre-1920s references to the sound of melancholy tunes (like "Texas Rangers") which, played on the fiddle, may have encouraged glissandos and blue notes, but such descriptions have eluded me. The practice is possibly post-1900 as well.