I'm 17 (still a virgin) and I just noticed a small, dark black spot on the lower part of my penis. It's not a bump - it's more like a scab. I messed with it (I know, bad idea) and it peeled back a little. I felt no pain and there doesn't seem to be any kind of wound underneath.
I suppose it's possible that one of my cats clawed me down there and that's where it came from... or maybe I got some ink or paint on it somehow... but does it sound lke a disease?