No, it's not because I'm emo.
No, it's not because I'm depressed.
Yes, I do in fact like the color, but no, that's not the reason.
Black Pixels require slightly less energy then white pixels (In Lighting, White has every color present, black is the absence of light) it saves money on people's power bill, it allows my page to seem more friendly towards energy use... Yeah, environmental science and art class left their marks on me.
I'm not some super pro activist or anything, but I mean, it's easy to set my backgrounds black, easier then going without power to save energy, so I mean, I guess I like doing the little things whenever I can.