It sounds like black mold from being kept wet and hot in a plastic bag. If so, it won't come out, and it isn't something I would continue to use. Your other diapers are probably fine, but I wouldn't continue to wash it with them.
Rebecca, married to J (12/07), DD1 Anna (11/09) and DD2 Ada born at home 7/12New Guy coming 3/14
ISO: Old style FB Elites with minky inserts