Greetings, What you have - as i can decipher from history and the picture that you have attached is - a condition called melanonychia. This is a harmless condition and usually occurs in dark and brown skinned individuals like Indians and Africans. This is nothing to worru about - however, in whites and European population this may be associated with certain significant conditions - so they usually consider nail biopsy - especially if the nail fold is involved. Howevre, in your case i dont think you need to worry about that.

Longitudinal melanonychia is the dermatological term used for your condition. It is highly recommended to see a Dermatologist. The underlying cause can be multi-factorial and very very rarely it can be an early sign of melanoma- a form of skin cancer. So do not delay the diagnosis. Hope it helps.