S.M.
I've seen purple mongolian spots that did indeed look like bruises. But I've seen brown patches that don't look like bruises at all. Since this poster is saying they look like tanning, I'd say that it's the type of birth mark my kids have. These don't look like bruises.
In any event, don't worry about it. I doubt you have a big problem. I did read once that if a person has more than a certain amount of them that it could indicate a disease. But I don't remember how many. It's quite a few though.