We have had more than one black spot on our hardwood. The hardwood Guy told us if the black is right through the wood, then it can't be fixed, outside of removing and replacing floor boards. it cannot be sanded out, but if it's just a surface black spot, then it can. Can you get a look from your basement, or is it finished ceiling without access to see the floor boards above? We moved into a house that had 2 rather huge black hardwood spots, possibly from a window leak or roof leak, but surprisingly most of it came out by sanding. I do not know if there is a stain remover specifically for that, or if it would work, it may also interact with any stain you use for touch up, so be careful and ask a professional.