What would cause toe and fingernails to turn black and fall off? no trauma, no pain.

Onychomadesis: Onychomadesis is defined as spontaneous separation of the nail plate from the nail bed, beginning at its proximal end and resulting in the shedding of the nail. It has been associated with a variety of clinical settings, including systemic illnesses, generalized skin diseases, and drug therapies. There have been a small number of reports in the older literature of idiopathic onychomadesis occurrin.