Think the Black Hole name came from the method they use even today to look for them: watching for black areas in space that look 'funky' and see how the planets/stars behave there. If they seem to have irregular paths and go faster at certain points of the path, it's safe to assume there is an Black Hole there that is pulling the planets/stars to such an odd paths.
This of course just my assumption how the name came to be, might be wrong.
As for knowing about the BH's, I'd say we don't know much, but we can assume a lot. We assume there's an huge mass in the center that pulls nearby objects to it, we also can assume that it is more of an grinder than an bottomless pit as it shoots out particles when it's "consuming" something, as to chew and spit it out again. I'm quite sure I saw/read from somewhere that they have a pretty good idea on how BH's are born too (big star imploding or something like that).
Again, this something I pulled from memory, might be wrong.