Black toenails can occur for a variety of reasons, there’s usually been some trauma involved which you will probably have noticed, causing blood to accumulate under your nail. I say you will probably have noticed it because it would normally occur after quite a hefty blow such as dropping a heavy weight on your toe or knocking it against something quite hard such as a step. But repeated minor trauma from tight footwear or running or walking very long distances can also damage the tissues under and around your toenails to cause bruising. If you have been told that you have reduced sensation in your feet then this could still be you even if you can’t remember injuring yourself.
A toenail bruise will have a black, purple or brownish discolouration caused by blood clotting under the nail. It may involve the whole nail plate or just a small portion. It can be very painful when it first occurs, medical intervention to relieve the pressure can help.
A bruised nail is nothing to worry about, the black will eventually grow out, the old damaged nail plate may even come off as the new nail grows through underneath.
People with naturally dark skin can have dark stripes in their nails, this is completely normal, usually affects several nails and more than one family member.
Rarely a black toenail can be due to a malignant melanoma, this will occur as a dark black stripe on the nail with the black discolouration present on the cuticle. Early diagnosis and treatment of melanoma improves the chances for a good outcome, so it’s important that all black toenails are seen by a podiatrist to rule out this cause.