With it being brand new, it could just be some funk on the belt left over from manufacturing transferring onto the pully. Like how some things have mold release agent on them.
I'd try to clean off the pulley with a rag and chemical (brake cleaner?). If you wanna live dangerously: If it won't scrape off with a knife or some sort of flat scraper, maybe hold a steel wool pad against it while it's turning? Be careful to not let it, or your fingers, get caught between the belt and pulley. Depending on how big it is, ignore or try to scrape or shave off the dimple. It looks completely ignorable. Probably some debris got into the rubber during manufacturing.