It is fairly common for people with dark skin to have dark stripes running down their nails
The streaks in the nail are caused by an overproduction of melanocytes in that area of the nail and can be common in children.
Other reasons why the nail pigment branding occurs is: The dark stripe maybe a result of a traumatic injury whereby the blood clot will resolve on it’s own over time, or due to medication, systemic diseases, vitamin deficiencies and other skin conditions which may also appear as banding in more than one finger. A doctor should review your medical history for the best advice.
If the line/lines were only present on one nail this could be a sign of skin cancer that effects the nail bed, also the appearance of the line would change over time, becoming wider or darker, please consult your doctor if this is the case.