Hi, I can't offer medical knowledge but I can tell you, I have a black line running down the middle of my big toenail and on my  small toenail of the same foot, I was referred to a consultant dermatologist, and was diagnosed with melanonichia, sorry not sure how to spell it. But any way I was told it was completely benign.and needed no further treatment. I would definately get it checked  by your gp. 

Hi, I can't offer medical knowledge but I can tell you, I have a black line running down the middle of my big toenail and on my  small toenail of the same foot, I was referred to a consultant dermatologist, and was diagnosed with melanonichia, sorry not sure how to spell it. But any way I was told it was completely benign.and needed no further treatment. I would definately get it checked  by your gp.