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Meatmarket guy here. It's ink, chances are it's from a big 'USDA' stamp that was placed on the beef when it was sold to the store as a primal cut (the ink bleeds through plastic a little bit), the tag was probably put on over the blue to cover it up, because the store knows people are scared of blue beef (they should have just turned your steak over, rather than hide the mark with a tag...). It's safe to consume the ink, and it won't be visible after you cook it.