Black toenails are common in those who are physically active like marathoners. A lot of times, this condition is caused by shoes that are too small. When the toenail continuously bumps into the front of the shoe, it causes a blood blister to form underneath the nail. This damages the nail and the tissue underneath it. For this reason, it is important to always properly trim your toenails. Excess nail that continues to bump into the shoe can cause chronic pain and discomfort.
Another cause for black toenails is trauma. When a large amount of weight is dropped on the nail, discoloration from bruising and blood blisters can also cause the appearance of a black toenail.
While nail infections due to fungus usually cause the nails to turn brittle and yellow, sometimes fungal infections can cause black discoloration under the nail. In some cases, odor and discharge are also symptoms of black toenail being caused by fungus.