Apparently the pinworm ova hatch around the anus then crawl back up to the large intestine and mature so they are motile. I did not see that they were dark colored but I guess that if they had ingested something dark in the feces they could look dark. You might have the Metametrix enteric test done to find out what other parasites are in your gut and that the pinworm Rx and herbs did not kill off.
MEBENDAZOLE (me BEN da zole) is an anthelmintic. It will kill some kinds of parasites. This medicine is used to treat infections of roundworms, hookworms, pinworms, and whipworms.