Would root canal therapy make the tooth black and brittle?

severe toothache --> dentist: there's bacteria and root canal therapy needed. But the gum of my other tooth (after a root canal therapy) , turned black with a black margin. It has a gold crown, I can' remember if the gum turned black before or after the crown. I was told that getting a gold crown is safe and will not incur black gum. tooth will turn weak the gum black after therapy? why did my other crowned tooth gum turned black? is there any way of treating the blackened gum? thank you !