Most commonly, black-striped nails are caused by symptoms known as linear melanonychia. According to the United Kingdom's National Health Service, these lines usually occur in African-American descendants aged 20 and older.

Linear Melanonychia is considered a normal nail color variation. This condition occurs when pigments in the nail known as melanocytes make excess pigments. This causes black nails.

Less common causes of melanonychia nails include:

Use certain drugs, such as chemotherapy drugs, beta blockers, anti-malaria drugs, or azidothymidine drugs

HIV

Laugier-Hunziker Syndrome

Lupus

Peutz-Jeghers Syndrome

Scleroderma

Another cause of black-striped nails is bleeding, which occurs when blood vessels under the nail are damaged. This condition is usually due to injuries, such as being hit by a blunt object or because of a hit.

Even more serious, black-striped nail marks can indicate melanoma, a dangerous form of skin cancer. Melanoma under the nail is known as subungual melanoma. One type of melanoma is known as acral lentiginous melanoma (ALM).