I received a frameset from a reputable Chinese manufacturer. When inspecting the frame there are visible black spots (in particular on the rear end triangle and bottom bracket) trough which no carbon fiber is visible. Do any of you know what this is? Possible black resin, filler or even carbon fiber?

Seems they didnt sand it away on purpose, because halfway the rearstays it's sanded down to the carbon. However I would love to do a naked carbon painting, which wont turn out so nice with these black spots I'm afraid.