Technically the fangs are venomous if the Black Dog purposely bites the target to enable the debilitation agent to travel through the wound; they're poisonous if Gams lets it enter his bodily system via inhalation, skin absorption, or ingestion (with a side implication that the fangs are edible).
But this misinterpretation between poison and venom persists across all fiction writing regardless of translation or author intention.