Is tarnishing (blackening) of silver a useful or harmful chemical change? Why?
Well, it's not useful, because you have to tell your butler to polish the silverware. The blackening, the tarnishing, certainly represents a chemical change.
The tarnishing of silver represents a redox reaction:
Silver sulfide (and silver oxides) are finely divided insoluble solids, and appear black.
There are many commercial silver polishes and silver dips. With good silverware, of course, nothing beats an old toothbrush and meticulous scrubbing. So tell your man to get on with it!