One of the most common causes of black fingernails (a.k.a. Melanonychia), which overall remains a relatively rare modern society ailment, is as a side effect of taking medication. In cases where the medication contains fluoride or Cytoxan, the person taking such medication may may develop black discolorations beneath some or all of the nails. Several other medications may cause this side-effect; it all depends, as with most prescribed medicine side-effects, on the individual taker's metabolic make-up.
A much more serious possible cause of black nails is melanoma. Complicating this diagnosis is the fact that a majority of African-Americans over the age of 20 normally develop longitudinal, pigmented black bands between their nails.
Doctors must rely on a variety of particular symptom watches in this case to differentiate between normal black line coloration and the possibility of an onset of skin cancer. Telltale signs include: a single toenail or fingernail only being affected; a family history of melanoma; and a band width of more than 3 millimeters.
Black Nails as Fashion Statement
On the voluntary side, that is in cases where women (and some men) deliberately color their nails black, the desired effect is usually one of edginess. Goth girls, whose look and heavy make-up tries to connect them with the darker and more supernatural side of things, often features heavy black eye shading and black make-up.
Similarly, when a celebrity such as Disney good girl Selena Gomez was suddenly seen sporting in public the fingernail polish tone of jet black, it was interpreted by celebrity websites as being an attempt to communicate that she was beginning to move on from her goody-goody, Disney star image. The Australian band Eskimo Joe meanwhile had their biggest hit in 2006 with "Black Nails, Red Wine," a song again celebrating the connotation of a girl with black nails.