Tooth has gone completely black under crown, after root canal a few years ago. Is this normal?

Teeth under a crown can be "black" for a number of reasons.  One, is that a root canal was done and not all the bacteria and blood was removed before the root canal filling was done.  Sometimes the top portion of the root canal can be removed (not destroying the integrity of the root canal, or needing it to be redone) and the tooth can be bleached.Another reason is simply bacteria getting under a leaking crown.  This can generally be removed with hydrogen peroxide quickly and easily.  Dentists often call this "black scuz".Your question, is it normal?  No, but this does happen and it is certainly preferably to avoid the teeth blackening over time.  If you're having a new crown done, any residual blackness can usually be covered over so that it does not negatively effect the final result of the color of the new crown. This sounds like what happened when your new crown was done.  I wouldn't waste too much effort on worrying about the color underneath.  As long as the root canal was properly done and the root tip doesn't show any sign of infection on the x-ray and your aren't having any pain, I would try to forget about it.