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I had a couple of large plastic metal plated serving trays I liked using. Turned out that something in the cured meat of my sausage was making black spots. Immediately thought of bubonic plague....LOL
It took awhile but I finally realized that something in the cured meat was reacting to the rusty parts of the tray. Soon as I quit using the trays I quit getting spots.
Don't know its your problem, but it was mine. It actually made black spots inside the casing on the meat, not the outside where'd you think. It reminded me of an old chem. class where we used silver nitrate and it burnt our hands black. <Shrugs>
The only time I have seen that color on meat was a reaction with Aluminum foil. A vinegar/wine marinated Meat was in a Steel pan covered in Aluminum foil. Next day the foil was pitted and there were black spots on the meat...JJ