I had a new dental crown fitted today, there is a black line visible at the gum line? Is this common with a brand new crown?

HI:It’s difficutl to give you accurate advice without a photo.  Please post a photo of the crown with the black margin.  Generally it take a few days for the gum to heal and cover the margin of the crown.Best,Dr. Maddahi

Hi!  Without a picture, it is difficult to answer.  That being said, some crowns have two layers:  tooth-colored porcelain on the outside and a metal alloy on the inside.  Sometimes, if the crown rests at or above the gumline, the crown will have a metal or "black line" appear below the porcelain line.  
Post a picture!