In Charge,
I cannot find where it says that. All I can find is where it says that an infection is indicated if the root of the tooth turns black.
My entire tooth turned black. Then it broke off at the gum line. The next dentist (translation: quack) I went to said that any time a tooth has a root canal it (of course) is dead and that it will turn black and that it why a tooth that has had a root canal needs a crown.