Tiny dark spot on dental crown (Photo)

Hi, about seven years ago I had 2 dental implants in my front teeth, and on them they put 4 dental crowns (meaning I don’t have any real teeth “within” them). Last night I noticed this tiny dark spot on one of them (see picture) and nothing can get it off. It seems like it’s part of the crown and has just always been there (but I know for sure it hasn’t). What can this be? Is there any way of removing it without completely switching the whole crowns?