Whenever something isn't going right within a shapeshifter's body, it seems to produce a black ooze in place of blood. The ooze is thicker than blood would be.
If a person is rejecting the Bite, the ooze comes out of every orifice: mouth, eyes, nose, ears. It is a signal that they are incurable and going to die.
Jackson bleeds black ooze when his body rejects becoming a werewolf but seems to stop when he instead becomes the kanima.
Gerard bleeds and vomits black ooze when his body rejects the Bite because of his ingestion of mountain ash.