Well thanks to a houseguest I just had for 2 weeks I now have big splodges of black on my beautiful guest room tiles!
my - now rather embarrassed - houseguest (close family member!) had been using some just for men gradual shampoo thing and ignored the white splodges in the shower. The next day they are now black!
The house is rented and I'm not looking forward to replacing the whole of the shower tiles 
Have so far tried
• mr muscle shower
• bar keepers friend (this usually works for EVERYTHING but hasn't even touched it!)
Do I try bleach???
Photo attached: