I have a dark vertical black line on my left thumb nail. is it melanoma?

Not likley: Melanomas are not linear. I suppose you could have a melanoma under the nail, but i still do not think it would look linear.

Unlikely: Melanoma under the nail usually runs horizontally, or the width of the nail, and not vertically.The parents of melanoma can vary quite a bit between patients, so if you are still worried or the black area does not appear to grow out of the nail you should visit your doctor for a complete examination.

Maybe?: Most likely a buise or injury; however, do not take a risk. Get checked out by a board certified dermatologist. If it is a melanoma, the sooner it is treated the better. Can't tell for certain without an examination.