Three causes for black toenails are:
- A too-short shoe (not enough room in the toebox)
- A toenail that comes into contact with the roof of the shoe too often
- A runner who uses his toes to grip too hard
Toes blacken because blood vessels under the nail break open, which spill blood (which looks black under the opaque nail) into the area between the toe bed and the toenail.
Solution: If the pressure is bothering you and you can handle more pain, press the end of a paper clip or safety pin, heated with a match, through the nail. This may be best done with a doctor’s guidance though. If the pain from the toenails decreases and doesn't bother you, no need to take action. Either way, the skin below it will heal, the nail will die and fall off. To prevent re-occurrences, address the cause.