There are several possible causes for black lines on nails, most are easily treatable conditions. It could be due to fungal nail infection, trauma to the nails which can cause splinter hemorraghes & taking some medications (chemotherapy & beta blockers amongst others). Persons of Black African or Black African-Caribbean descent are also prone to getting these lines due to melanocytes making less pigment in the nail so the nail bed gets darker. The most serious condition is subungal melanoma, however this is not common (1% of all melanoma diagnoses) and it only usually occurs on one nail.
Whatever the cause, you need to see your GP so that it can be diagnosed and the appropriate treatment prescribed. Your GP will be able to calm your worries and sort it out for you.
