Sometimes this can be an area below the edge of the crown where natural tooth is exposed and decay is present. But in a lot of cases, it is just the crown itself.
Certain crowns are made with a metal base onto which porcelain is added to give the appearance of a natural tooth.
Over time when the gum recedes, this area of darkness that wasn’t visible is then exposed. It doesn’t mean the crown has failed from a structural/functional standpoint but depending upon how visible it is patients sometimes choose to have these are replaced because of their appearance.
With the introduction of natural looking and strong (zirconia) crowns without any metal, Dr Cordes can place all ceramic crowns anywhere in the mouth. This eliminates black lines completely!