It's black with a silver background (7)

I believe the answer is:

tarnish

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But I can't explain it at all! Could you help me understand?

(Other definitions for tarnish that I've seen before include "Lose lustre; sully" , "Become dull or blemished" , "(Of metal) lose lustre" , "Stain, metal or reputation" , "Discolouration of metal surface" .)

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