Stainless steel woks are basically iron-based and infiltrated with chromium to make them less likely to rust. When making soup, the temperature of the pot is very high, which still oxidizes it. Strong oxidation will cause it to turn black, but it is not toxic and harmless to the human body. Just use a stainless steel wire ball to brush the inside clean. No amount of brushing will make it oxidized. The reply is bright, don't worry about it outside.