I have a reddish/brown mark that looks like bruised blood under my skin on my toe. what is it?

Hematoma : If the dried blood is under the nail it would be called a sublingual hematoma. If it's under the skin it's called a sub dermal hematoma. They usually are caused by blunt trauma or repetitive trauma to that toenail or part of the skin. Usually a new nail grows out in 4 months, it's cosmetic if anything. If it is really painful your foot doctor can drain the dried blood in their office.