To me this looks just like environmental damage. Cupro-nickel coins (including modern US clad coinage) tend to go like this when they are buried in the ground. I reckon someone found this dime with a metal detector and put it back into circulation.
Trisha, first welcome to CCF. Second I agree with @numisrob that your dime has most likely suffered from environmental damage to get that dark. However, you may find the following link of some interest, as it was a deeper dive into similar coloration on Jefferson nickels:
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