Metal jewelry turns black after sweating in water. How can I change it back?

Don't use toothpaste, because toothpaste contains friction agent, which will make your necklace dirty and dark. Dilute hydrochloric acid is ok, but not for too long, which is dangerous. After all, it is a corrosive liquid. You can try this method: first prepare half a pot of water, add a little white vinegar, and then put the necklace in the pot for ten minutes, which is very useful. If there are other ornaments or pendants on your necklace that are inconvenient to heat, it will be very useful. Put it in a vessel, add water (to submerge the necklace to a higher degree), soak the necklace for 20 to 30 minutes before wiping it. If it's not rust, it's sludge or something like that. I suggest you wipe it with alcohol. After cleaning, if you don't bring it often, put it in a small box with several bags of desiccant (in some foods) to prevent it from rusting again because of the humid surrounding environment. If it still doesn't work after trying, then.