Blast-Furnace Hellkite’s last ability applies to any creature that is attacking one of your opponents, even if another opponent controls that creature.

When you cast a spell with offering by sacrificing a permanent, generic mana in the sacrificed permanent’s mana cost reduces only generic mana in the spell’s total cost. Colored and colorless mana in the sacrificed permanent’s mana cost reduces mana of the same type in spell’s total cost, and any excess reduces the spell’s total cost by that much generic mana. The total cost of the spell can’t be reduced to less than {0}.