Melanonychia is a black or brown pigmentation of the normal nail plate, and may be present as a normal finding on many digits in black patients, as a result of trauma, systemic disease, or medications, or as a postinflammatory event from such localized events as lichen planus

Lichen planus

Lichen planus is a chronic mucocutaneous disease that affects the skin, tongue, and oral mucosa. The disease presents itself in the form of papules, lesions, or rashes. Lichen planus does not involve lichens, the fungus/algae symbionts that often grow on tree trunks; the name refers to the dry and...

 or fixed drug eruption.

There are two types, longitudinal and transverse melanonychia. Longitudinal melanonychia may be a sign of subungual melanoma (Acral lentiginous melanoma), although there are other diagnosis such as chronic paronychia, onychomycosis, subungual hematoma, pyogenic granuloma, glomus tumour, subungual verruca, mucous cyst, subungual fibroma, keratoacanthoma, carcinoma of the nail bed, and subungual exostosis.