I like the idea of a dark floor. It probably should have a little tonal variation so that it hides lint, etc. I think dark floors ground a room.

We did a dark floor in our tiny MB and it looks really great! Doesn't make room look small, in fact, it makes a nice impact (IMHO).

I have a powder room with dark gray/silver/charcoal/black Ostrich Gray Slate floors, white fixtures, white trim, white walls. I absolutely love the look. Personally, I'd probably keep the walls a lighter color, but white is certainly not a necessity. I think you'll be fine with dark floors.

I did a dark floor in two tiny bathrooms and I love it.

We have dark wood (!) in one bathroom and black hexagonal tile in the other. Both teeny tiny BR's. Looks great. You'll be fine.