Belly button question

Just out of curiosity, not meant to sound rude/judgemental. . I've seen all kinds of belly pics with all different shapes and sizes but the only thing I can't figure out is why some people have a dark circle above/below the belly button. The only thing i have come up with is maybe scaring from a piercing or a stretch mark that occurs. Maybe it's part of the linea negra? Sp? I'm just looking for some knowledge as I can't figure it out on my own. I'll try to find a picture of what I'm trying to explain. .