Medically reviewed by Susan Bard, MD A black line on the nail could be normal, such as when it is just a color variation.

Soon after completing the grueling challenge, he developed a black toenail — a painful sign of foot trauma or poor footwear. Despite the injuries, Welsh said the experience was worth it.

Experts have warned that what may look like a toenail injury could actually be a skin cancer symptom. The main cause of melanoma is ultraviolet light, which comes from the sun and is used in sunbeds.