Black fingernails may be the result of a hematoma, according to Dr. Melissa Conrad Stöppler writing on MedicineNet. A hematoma is a pool of blood under fingernails that is usually the result of some type of trauma.
A drug called Aralen, which is used to treat parasitic infections and certain autoimmune diseases, may also cause fingernails to turn bluish-black, states Dr. Wingfield E. Rehmus writing on Merck Manuals. A drug called Zidovudine, which is used to treat HIV infections, causes black and brown streaks. Those streaks can also appear in someone who has AIDS but does not receive Zidovudine treatment.