1. Most commonly it is due to the toot underneath being dark and showing through the gum. This makes the gum have a blue hue. The usual treatment is to replace the crown, tucking the margin of the crown further underneath the gum to cover over the darkened root and/or bleaching the internal surface of the root via the root canal to lighten the tooth.

2. The gum sometimes can become tattooed by the metal in the existing crown. In this case simply trimming the tattooed gum away can lead to an overly long tooth. The better but more complicated way is to extrude the tooth ( make the tooth grow down). The gum grows down too and the excess can then be trimmed back to match the level of the adjacent tooth, so removing the dark gum. A new crown would then be made.

Hope this helps.

Regards