Thanks for the info on the meaning of the word "blues" in lyrics. It looks like "I've got the blues" meant pretty much the same thing in the first two decades of the 20th century as it means today, i.e. "I'm sad or depressed and maybe feeling beaten and abused by the world." And it apparently has always meant, that whether it's used in a blues song or in some other type of song or in any other spoken or written language. Though not exclusive to African-Americans, that feeling has been a major part of their experience since the first slave ships brought them here.

I can also accept that "singing the blues" can be similar to "crying the blues," i.e. complaining about the aforesaid emotional state and/or its causes. But I'm still having a problem with the cheerful nature of the music itself, often with no lyrics at all, and the idea that people would have called that style of music something that means "the sad feelings." That applies not just to early blues tunes and other compositions with "blues" in the title, but also to the earlier unpublished African-American music that was apparently the source of that style. I think the music, at least in those early and formative periods, was more often happy party music. It also included some sad-sounding music with sad lyrics, but even then it wasn't as depressing as a lot of other music of that time.

Consider "I Got the Blues," by Antonio Maggio, a dance tune with no lyrics, published in New Orleans in 1908. Listen to it at https://www.youtube.com/watch?v=ta3zrj1BXxY. Does that sound like he's sad?

Or "I'm Alabama Bound," sub-titled "Rag-Time Two-Step, also known as the Alabama Blues" and "Respectfully dedicated to all those who have the blues," by Robert Hoffman, published in New Orleans in 1909. Listen to it at https://www.youtube.com/watch?v=GH_8x3ezI2g

If there's any connection between that music and feeling sad, it's that the music can lift your spirits and make you stop feeling sad. But that can be said about any music.

It's unfortunate that the term "blue note" is a later development, because that's a case where it would make sense. I can see how a blue note could be described as an individual note that's sad or depressed, even if it's used in a cheerful melody, and it would make sense to use the term "blues" to describe a composition featuring blue notes prominently.

And I could see using the term "blues music" to mean the music of African-Americans, because it would mean the music of a people with good reason to be depressed all the time, even if they overcome that depression with their music. But blues isn't the only kind of African-American music that the general public knows about, and wasn't at that time, right?

And what about jazz? No ideas about where that word came from? I don't have a clue. It's interesting that the words "blues" and "jazz" have similar sounds. Could "jazz" be a corrupted spelling of some plural noun?