Vertical black lines under the fingernails are often the result of a splinter hemorrhage, according to WebMD. Splinter hemorrhages occur when the capillaries of the tips of the fingers bleed between the fingernail and the tissue underneath.
Splinter hemorrhages also may be a warning sign of infective endocarditis, in which the valves of the heart and the endocardial surface of the heart become infected. Splinter hemorrhages are rarely the only manifestation of infective endocarditis. This serious medical condition is often accompanied by fever, muscle aches, heart murmurs, tender nodules on the fingertips, retinal bleeding and a stiff neck, according to Medscape. The black lines that are associated with bacterial endocarditis resemble wood splinters. Antibiotics are prescribed to treat this condition, and after the condition is resolved, the bleeding under the nail stops and the black lines go away.
Black lines under the fingernails may also be related to longitudinal pigmented bands, which are considered normal in dark-skinned people, according to the American Family Physician. This condition, however, must be differentiated from other conditions such as subungual melanomas, which is a type of malignant melanoma that develops underneath the fingernails. To determine whether longitudinal pigmented bands are benign or malignant, a biopsy of the affected area must be performed.