Copper oxidizes, where the shielded wire has had the shield stripped away, the copper will change color because of oxidation. Just like a penny will.
If it's black where the shield has been pulled away, it could be that it arced, and oxidized during that process, which would make it turn darker.
If you come across a large amount of copper that has been burned, there are ways to clean it easily, without too much expense, so that you will not incur a fine. I believe however, most places that fine for burning wire do so when they are able to catch the person in the act. But that burnt wire will not bring you a fine, it will just make it impossible for you to sell it as they scrap yards that purchase it will be fined for having it, and will not buy it from you.
Scott
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