What about etymology? It's been touched on here, but not directly addressed, and I think it has bearing on the topic.

Does anyone have evidence to show why the term "blues" was selected when it became necessary to have a name for that kind of music?

Most people today think of blues music in connection with blue=sad. Joseph cited early examples of songs that appear to have made that connection. But blues has always included a lot of happy dance music and even comedy, contrary to the direction taken by the NTV show that Helen linked to. There's certainly nothing sad about "Dallas Blues," "Weary Blues," "West End Blues," "Jelly Roll Blues," or many other early pieces that used the term. And there's a huge body of extremely sad pre-twentieth century vocal music that would have merited that name more if it was in fact a reference to sadness.

The color blue has many other associations, such as "off-color" or "pornographic," and the "blue note" that characterizes blues music. We don't see those uses in pre-twentieth century literature, as we do the blue=sad usage, but I wouldn't expect either of those to be used in the literature of that time even if it was common in the vernacular. And the fact that blue=sad was used in 1798 doesn't necessarily mean that it was the most common association with that color in 1915.

A blue moon doesn't suggest sadness, just something unusual or rare.

The nursery rhyme "Little Boy Blue" suggests that at one time the color blue may have been associated with things going awry. That sounds like it could be connected with the idea of a blue note.

Kurt Vonnegut said that when he was a boy his mother's generation used the term "blue Monday" to indicate laundry day. That term has come to mean something completely different and to be associated with sadness. But laundry day wouldn't have been a particularly sad one for housewives in the late nineteenth century. We know they used "bluing" or "laundry blue" as a fabric whitener. That's more likely to be the origin of the term for them, and the idea of making dirty things turn white would give the word blue a bright, happy connotation.

Oddly enough, blue has also been associated with black, the exact opposite of laundry bluing. In the past, the word black was often used to mean anything dark, such as dark-skinned people or dark brown bread, and blue-black sometimes indicated true black. And then of course there's the association with fidelity, as in "true blue."

US soldiers wore blue uniforms from the Revolution till just before World War II, and soldiers were called "the boys in blue," a term later used for police in eastern US cities.

Blue is often a comforting color, the color of the sky in good weather, and in that context it's more naturally associated with happy rhythms such as those of early blues rags.

But blue can also indicate exposure to extreme cold, or bruising. It was once common to speak of a person being "beaten black and blue." Kerouac suggested that "beat" culture was the culture of people who had been beaten so thoroughly by the system that they had come to embrace the beating as a way of life. Could that idea have been around earlier, with a suggestion that being really beat made your skin turn black and your music turn blue?

N.B. I'm just asking. I don't know where the musical term "blues" came from, and I don't know whether any of the associations with the color blue that I've listed have any bearing on it. But I'm real curious about it.

And while I'm at it, what about "jazz"? A long time ago I listened to a radio discussion of jazz origins in which everyone seemed confident that they knew where that term came from, and they implied that it was pretty obvious if you think about the term and the culture that produced the music, but they also indicated that it would be too indecent to say on the radio. Any idea what they might have been talking about, and whether there's any evidence to support it?