Dark lines or spots at the gum line are caused due to gum recession, or formation of tar, because of excessive smoking or using tobacco products.
Obviously there are many reasons, but black lines are not to be overlooked. You got to see, if the dark line is a layer, or your gum has started to reveal the lower portions of your teeth. If it’s a layer, then probably professional scaling or cleaning will help.
But if the gum line seems to have come down from the initial level, thereby exposing the dark layer, then you must immediately see a doctor!