The first time you develop a black toenail, it can be considered as a sign that your training has moved into a higher gear. The natural tendency is to blame your shoe for the injury but this is not always the case. Pressure exerted by the shoe may hurt a lot but it seems that the actual cause of your blackened toenail is pressure exerted from beneath your toes during the course of your walk.
The combination of pressure from the shoe and sock with the persistent jarring of each step produces an impact between the toenail and the tissue surrounding it. Repeated impact over a long period of time, such as a 50 mile walk can result in damage to the surrounding tissue which may become inflamed. Broken capillaries in the injured tissue result in bleeding which eventually turns the tissue purple, blue and black. Additional fluid accumulation exerts further pressure and causes additional pain. The increased pressure may eventually result in the toenail becoming detached from the nail bed. The blackened toenail will eventually drop off but this process may take up to several months.
The longer you walk the greater are the chances of the injury getting aggravated. If you notice any kind of discoloration after a walk or a run you should ease off on the routine so as not to aggravate the injury. You can reduce the chances of a toenail injury if you wear shoes with a gap of at least ½ an inch between your toes and the shoe.
If you do develop blackened toenails, it is best not to take any action if the pain is bearable. The injured portion of the toenail will eventually fall off and your toenail will return to normal. Wait for a 24 hour period before deciding whether the pain is manageable or not. If you detect redness or if the pain gets unbearable, then you should consult your doctor as you may have an infection.
answered by M W