This dark line or discoloration on the nail can be a common condition called melanonychia, basically, extra pigment in the nail plate. In many people, especially those with more melanin in their Diagnosis and management of nail pigmentations ScienceDirect Melanonychia Striata Longitudinalis (MSL): Pigmented bands Academic Dermatology of Nevada Nail changes can reveal major systemic disease, from cirrhosis and endocarditis to melanoma., Splinter hemorrhages + fever murmur should make you think of infective endocarditis., Single Melanonychia: Clues for Diagnosis and the Importance of Histology Next Steps in Dermatology