Will root canal cause black gum line?

Most of the time a root canal treated tooth does not significantly change color, but CAN.  Zirconium crowns do a good job of masking this darkness, so I think it is a better choice than a PFM.  I usually do an eMax crown in the anterior, as it has similar strength properties but more esthetic than zirconium.  Some will do a zirconium core and a porcelain or emax overlay, but this can lead to breaking of the porcelain at the interface of the core and porcelain.  A monolithic eMax would be my first choice.

Black lines at the gum line are common due to darkening of the tooth after having an root canal treatment. However, with the proper restoration, i.e. a Zirconium crown, there is a high likelihood that we can mask the black line. You definitely want to avoid a PFM if black lines are of concern to you. Good luck!

Typically a root canal treatment does not cause a tooth to darken.  A tooth can darken, however, if it has abscessed though even this is rare.  While Zirconium is a very strong crown, I believe most people are using Emax for their anterior crowns.  It is also very strong but, at least with my experience, is a better looking crown.  Good luck to you!