I've seen pictures of a rather dark cockatiel, and I've also seed, in person, a rather dark colored cockatiel. It's hard to say if the one in pics was actually black but both cockatiels appeared to be rather dark grey (charcoal grey?), not actually black.
I don't have pics of the male tiel, but here's the pics of other tiel (taken with a phone so bad quality)


Now some say it could be either a silver mutation or an olive mutation... yet another says it could be due to selection. The point that was made was that it would be impossible to have a truly black cockatiel due to the feather structure (but I don't necessarily agree with that).
Other than that there really isn't much information out there....