My toenail turned black after hiking, what does it mean?

Three weeks ago I was hiking on hilly terrain which I'm not used to, as I live in Florida. i wore my broken-in running shoes. My toes hurt for 2 weeks. When I removed my toenail polish, the big toe on my right foot is mostly BLACK! I doesn't hurt anymore, but what does it mean? Why didn't this happen to any other toes?