Does 925 sterling silver turn black?
Sterling silver turns black because it oxidizes. There is a neat little chemistry trick to remove that oxidation (mild oxidation) without polishing off silver.
We will assume that you have a silver dish.
Line the bottom of a container with aluminum foil. Boil enough water to cover the dish if placed in said container.
Slowly add in Sodium Bicarbonate (baking soda) to the boiling water. Slowly, as it will boil over easily.
Once it is mixed, pour the solution (while still boiling) into the container over the silver dish.
Allow it time to do its thing.
I think the oxidation is silver sulfide. Aluminum has a higher affinity for sulfur than does silver. Where the oxidation contacts the aluminum there will be a minute electrical current and the sulfur will be transferred to the aluminum, saving those silver atoms while getting rid of the issue.
This will be noticeable when you take out the dish and observe the foil. It will appear tarnished and the dish less tarnished.
I polish silver like this often.