What is the cause of dark gum line around tooth after root canal? What options are there to correct this?

If the crown is no metal as you mentioned the darkness should not be from metal shining through as happens with metal containing crowns.  After root canal treatment darkening of the tooth is common.  There are two reasons that the dark area would be showing:1.  Gum recession, which is most common, and exposed the dark area of the root.  If this is the case a new crown can be made to cover the dark area again. 2.  Thin gum tissue- if the gums cover over the crowns margin and there is no recession presence thin gums may be more translucent and cause the underlying dark root to shine through. Discuss the cause with your dentist.