Pete - As I told you in a e-mail, the strike is toned, look at the bottom part and you see the toning is a differnt color, also Silver can and does turn many differt colors when it starts to tone, the shades can be bule, purple, gold, red etc. not just black, some silver coins that have toned are outstanding in looks others are ugly.
The strike could have had a substance of some sort on it in the minting process but it isn't an error.
I could tone several common strikes to come up with what you have but that would just be wasting the st4rike.
Walt