Does Shadowheart have that absolute shield or gear? Some of those modify the behavior of heal/bless. So if she casts on someone who is not branded by the absolute, then it becomes bane.
How to get the gear to work:
You need to be branded with the absolute's mark in the goblin camp. If you talk to the priestess she'll offer to brand you. The brand is there after you kill her too - but I haven't tried to brand myself/party members, not sure if that's possible.