We’re having our baths redone. Both baths have the same shower wall tile and the same shower floor tile. Both have a white grout on the walls, and a medium gray grout on the floor. One shower is fine. The other shower floor has developed black areas on the grout. It has not been sealed. The contractor had the surface grout in those areas removed and regrouted. The black appeared to be surface only. We do not use this shower so it can be as dry as possible. Well. The black came back. The contractor is flummoxed as to why this is happening. Any input/advise would be greatly appreciated.