Black gunk around root canal plug

My 13 yo son had root canals done on his two front teeth early last year. I brought him to the dentist the other day to get a chip fixed (in one of the front teeth) and the dentist saw on the xray a bunch of black gunk surrounding the plug. He cleaned it out and put new stuff in. He said the other front tooth has the same thing going on.

My son’s front teeth have been discolored since his root canal procedures. I assumed it was because of the damage and trauma to his front teeth but the dentist said it’s from the black gunk. As soon as he cleaned out and repacked the first front tooth, the tooth was instantly white!

He and the dental assistant said that the black gunk is not typically seen after a root canal.

Did my son’s endodontic screw things up?