My bathroom floor is black.
I think it's ok to have black gloosy wall tiles , it's rather easy to maintain.
But black floor tiles is harder , it will leave white residue on the floor due to shower gel and shampoo use.
No matter how I scrub and clean , it will re-appear 2-3 days later, coz we bath at least twice a day , thats like 4 times a day ( me & hubby )
Perhapes grey wld be less obvious ? Hmmmm ...?
Anyways,regarding ur parents , at the end of the day , it's your home. You shd be able to do anything you wish. Or maybe limit the all black only to your MBR toilet.