Boy, this is resurrecting. I use cream of tartar and water enough to cover the burn. Warm slowly to almost a boil and then turn off heat and let sit overnight.
I've never really blackened a pot, but have browned tomato sauce enough that it tainted the whole batch. My remedy worked.
People also say coke/soda works and rubarb works. Never tried those. (don't drink Coke - Pepsi only LOL! And rubarb in Florida - good luck!)