It's all the emo kids.
I specifically looked for black at DLR today. In the morning, at DCA, I didn't see much black. In the afternoon at DL, there was more black.
I would estimate that about a third of the people dressed in black (that I saw) were teenagers.
I also noticed that many of the non-teenage people wearing black were of one of two specific ethnic groups. I hate how that sounds, but because of this thread, I was paying attention and looking for trends..