Toenails that turn black can indicate nail or toe trauma, fungal infection or, in rare cases, malignant melanoma, according to FootVitals. People with discolored toenails may notice increased pain and swelling or, if the toenail becomes infected, an unusual odor or discharge.
When running, a runner's toes may repeatedly rub against the tops of their shoes, causing blisters to form underneath toenails, explains About.com. Ill-fitting shoes, poorly trimmed toenails and excessive sweat inside running shoes can cause nail or toe trauma. To prevent injury, runners should purchase comfortable running shoes, trim toenails often and wear wicking socks to prevent sweat from building up.
In most cases, the black toenail falls off on it own to allow a new nail to grow, notes FootVitals. Sufferers should seek medical attention if the injured area becomes infected as this may indicate osteomyelitis, a serious bone infection.
To conduct a full inspection of the nail bed and toe bones, physicians may remove the damaged portions of the nail or use a needle to puncture the blister and drain the fluids present, reports FootVitals. At home, those with nail trauma can soak injured toenails in Epsom salt and warm water and then apply a topical antibiotic to the injury site. Toenails typically grow back within a few months but may appear uneven or irregular when compared to other toenails.