I would not do black, but maybe a medium grey. Black is very harsh look. Try to look at grout colors on houzz. Most of the ones that look like they have a dark grout are really just a medium color.
Our bathrooms have 4x4 square white tile on the floor. The grout is a light brown. It's probably stained that way, and was originally lighter. If I stained the grout black, do you think that would give it an updated look? Or at very least a more even look?
Historic colonial - 1890s, with two standard four piece bathrooms, nothing fancy.
Eventually I'd like to retile, but it's not in the budget right now.