Black marks on nickel coins

Is it possible to find a pure nickel coin with black spots?
From what I know nickel cannot oxidize. So if I have a nickel coin with black spots, they are dirty and so I can tru=y to eliminate such dirty

Where did you get the information that nickel can't get oxidized?
Have a look at the Galvanic series and you can easily see for yourself that this is in fact not true.

Also black nickel compounds should be derived from sulphides either through the air or your bodily secretions .