How can i get a 24-bit tileset with black in my tileset?

You can't get a 24-bit color tileset.
You must make a 256 color tileset.
And I don't know how to make a black tile.

Ooo, it's simple... Either make it so that the black color on the palette isn't one of the colors on the 1st row (this could get complicated). Or take the lazy way out and make the color slightly off-black like #000001 or something like that.

Well I always made my black number 2 on the pallete. Because JJ2 treats the 1st colour as transparent (I think). I don't know if this is accurate, but it worked for me, so you might want to try it.

Also, even if you change the black in spot 1 to another colour, it will still treat it as transparent. You need to put the black colour somewhere else in the pallete.