Pretty much what the title says. The way holocaust is described, I would interpret it as bypassing any and all defenses or resistances, but I'm not sure if I am correct.

Example: Target has some dark blessing or whatnot that makes him IMMUNE to fire damage. Psyker fires off (no pun intended) Holocaust. Does it effect the target regardless?

You are correct, it bypasses everything. The errata further buffs it to state, "Wounds caused by this power may not be healed by any psychic powers."

It's one of the more broken powers and a lot of GMs ban it.