I swear my 18 y/o DS's back is like this. He's severely disabled and just thinks I'm hurting him when I've got one, so I leave it alone.
Imagine that, it's like they're winking at he sometimes, but I don't want to hurt him. He wouldn't understand me explaining it. So I just leave it be.
He's been on antibiotics for acne for years, but his back is a different story, massive, ripe blackheads.