I'm not sure what you mean when you say that "the last song downloaded in black."
If you're saying that the entire UI suddenly turned black, it might be possible that you accidentally engaged "invert colors" using method two of the two methods described here. If that's what happened, you can undo it by triple-clicking the home button.
But I'm just taking a stab in the dark. I actually do not understand your query at all and it is quite likely the tip I offered won't help.