The good news: As long as you're not in pain, it will probably heal on its own.
Q: One of my toenails is turning black-and-blue from running. Should I be concerned?
A: Bruising or slight bleeding beneath a nail is a common runner's injury. Your toe may be bumping against the inside of your sneaker as you run, creating a blood blister under the nail. Check if your shoes are the right size, either by visiting a special running shoe shop or by measuring the space between the tip of your longest toe and the end of the shoe—you want a thumb's width of room. Consider reducing the amount of running you're doing or lessening the intensity so that your toenail can heal without repeated stress.
As long as you're not in pain, leave the nail alone. It may eventually fall off, and a new one will grow. If your toe is throbbing and sensitive because of an accumulation of blood under the nail, your doctor may need to make a hole in the nail to relieve the pressure. Do not try to do this by yourself; it could lead to an infection and more pain.
I also encourage you to see a doctor if the dark spot covers more than a quarter of the nail or if it doesn't go away after six months. Sometimes a fungal infection will turn a toenail very dark and thicken it, which requires an over-the-counter or prescription antifungal medication. In super-rare cases, a black spot beneath a nail may be a skin cancer, so it's best to let your doctor assess the discoloration.
Health’s medical editor, Roshini Rajapaksa, MD, is assistant professor of medicine at the NYU School of Medicine.