My dog has a little light pink bump with a tiny black spot in it on his belley right above his penis. What could be wrong?

Bumps are hard things to identify. Especially without pictures. It could be anything from a bug bite, to a splinter to an ingrown hair.

Can you tell if the black dot is a foreign body, or just a mark on the skin?

I know mine get lumps and bumps quite often, and usually they aren't anything to worry about and disappear over a day or 2. They can even get pimples and blackheads on their skin just like humans.
I'd put my money on a simple mosquito bite though.

If it doesn't look like it is getting smaller over the next day or 2 then book an appointment with your vet.

How old is he and is the spot new?

First thoughts are tick (if new) or belly button (if not).