I stubbed my toe and now it's black and swollen. Doesn't hurt much but I have a dance tomorrow. What to do?

See a doctor. Your symptoms would deem avisit to the podiatrist to see if the toe is fractured or how severe the injury is if it is black.
Be more careful. Unless you have peripheral vascular disease, ice is your friend & will help. Blood under the nail should be released it as soon as possible after the injury using a hot paper clip to make a hole in the center of the nail or better yet - have a trained health professional do it. Ibuprofen and Naproxen will help with inflammation. Let pain be your guide. If its hurts too much to dance then don't.