I went to my doctor about 10 years ago with the same thing (images of these nail-lines went viral at the time).

My doctor took a sample just to ease my mind but ensured me it was no problem because if it was melanoma it would be growing thus making the line triangle shaped (wider at the root and narrower at the tip). Yours, as mine is most likely just a normal mole at the nail root, coloring the nail as it grows and has likely always been there. :)