"... meaning a note that sounds sad or off-color or a departure from the norm..." Bent notes weren't a departure from the norm in black folk music, they were normal. Listen to a bunch of versions of "John Henry," for instance.

It didn't "make sense" for writers of the 1910s who didn't know all that much about black folk music in general to associate a characteristic of black secular and religious folk music in general with blues songs in particular, given that as of about 1908, only a fraction of secular black folk songs had been blues songs. But things often happen that don't make sense.