The black or gray outline you see is an indication that there is metal underneath the porcelain of the crown. These PFM, or porcelain fused to metal, crowns were very commonly used prior to the advent of the more modern materials we now have at our disposal.
The black line was probably not visible when the crown was first made and placed. Over time, irritation and inflammation from the metal has caused the gum tissue to recede away from the margin of the crown, and exposing the dark line you see today. At Emerson Dental we only use metal-free porcelain/ceramic based crowns, eliminating those unattractive dark lines.