Yes the outer layers of a modern dime are composed of cupronickel (75% copper, 25% nickel) alloy which, while not as reactive as silver, will tone over time leading some coins to have a dark appearance. It is normal, and nothing to be concerned about. It can reduce the marketability of a coin, if very dark, but is generally considered neutral by collectors. The innermost layer of those dimes is copper. Overall composition is 91.67% Cu and 8.33% Ni.
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