I didn't mean to say that a blue note sounds like a departure from the norm in a musical style that regularly features blue notes. That would be absurd.

I meant that a blue note sounds like a departure from the norm as compared to the other notes in the scale that it's used in. The other notes have a common relationship with each other. The blue note has a different relationship with the adjacent notes, and that difference can be heard, and the nature of that difference in sound makes it easy to think of the note itself as being sad and droopy, or dirty and off-color, or out of place like the sheep in the meadow and the cow in the corn.

And I didn't mean to say anything about black folk music. I'm not convinced that the name for blues music originally came from the perception of certain black folk music as a particularly sad musical style. That's not to say that the word "blues" didn't later come to be associated with a sad-sounding musical style that has much in common with some earlier black folk music. But the earliest published compositions with the word "blues" in the title have a different sound, and those compositions appear to be concurrent with the earliest use of the word "blues" to describe a musical style.

Often we don't understand the logic of something that has happened, especially if we assume that the facts we have are the only relevant ones. But there is always a logic, and the failure to see it is a clue to me that I don't have all the facts.