Hello...  The description u have provided highly suggests the diagnosis of melanonychia,  where there will be excessive deposition of melanin pigment in the skin underneath the nail.. Usually it's being and doesn't pose any harm,  unless aesthetically demanding.  But for diagnosis and removal if required u can consult an appropriate doctor..

Thanks for the query. Thats a very limited description to make any comments. I suggest you to see a dermatologist for a diagnosis and treatment plan. Delaying may not be a good option unless you know what the diagnosis is. Hope it helps you.