Oh dear , sounds like a slight acidic based etch, not sure a cleaner will do this , may have to mechanically remove ,but this can open a can of worms as black granite can sometimes be resin bonded,dyed or colour enhanced, I used to use a dye called Zimba black to re dye these, after re etching the whole top!!! Don't advise this , can they speak to their original supplier to pop out and re surface or just advise . Unless it is a really old top.
Acidics will always be an issue even with an impregnator as these do not hold this type of contamination unless the granite is fully acidic resistant itself.
Wayne, I would not touch it,
