Are you just trying to seal the concrete, or are you more interested in making it black?
It's a bit late now, but I think there are several concrete pigments you can get and add to the mix to change the color of the final product. Black might be hard to do, but I'm sure you could get a dark grey fairly easily.
You may be able to use a concrete stain to change the color of cured concrete, but I don't know what they use, and I wouldn't do that without sealing it as well.
I think people have used drylock with some success, and I'm prone to use epoxy for everything. Just for coating concrete, you may not need much epoxy, and you could possibly thin it as well - check the instructions, I think acetone usually works. Also, sometimes you can find tints for epoxy, so you could easily tint it black.
I think I remember reading somewhere about using concrete for ponds (a book, no idea which one), was that after it was cured, to fill it with water, add some vinegar, let it sit, drain, repeat; that way most of the easily soluble minerals would leach out.