Why do I have blue gums under an old root canal? (Photo)

After root canal there is no more blood supply to the tooth, your tooth changes color. You have placed a crown over the root cal tooth, in the beginning, you probably did not see that, but now due to recession of your gum, the color is obvious. It is best to see your dentist to check the tooth for correct fit of your crown. If the fit is satisfactory you should not be worried, however if you do not want to see the obvious bluish color you may need a new crown without metal and possibly gum treatment.Best of luck 

Hi:Once you get a root canal the tooth and roots of the teeth darken over time, this results in the bluish tinit to the gum.Best,Dr. Maddahi

It looks like you have a metal base under that crown. That can show through the gums causing that coloration.