I hope there is not some blatantly obvious answer to this question, if so, my apologies.

Under the description of the "Cult of Blood" option for a DCA, it states that:

"The Death Cult Assassin automatically confirms all Righteous Fury rolls when inflicting damage with a melee weapon."

What is the meaning of "confirms" in this context? It cannot mean that all melee attacks made by the DCA automatically cause righteous fury damage, can it? That would be amazingly overpowered (especially when Cult of Death offers a RF on a 7+ once per turn). The rest of the description is reasonably clear, but this "confirms" makes little sense to me. If all it means is that whenever the "0" is rolled, you say, "Yep, i really did roll that 0, time to roll me another!" What is the point?

The point is that you'll never ever fail to confirm RF, every 10 you roll means another die. It seems rather dominated by "Cult of Death".

Check the main rulebook. When you roll that first 10, you have to roll another attack with your WS or BS, as if you were attacking again. If that "attack" fails, you don't get the RF, just 10 damage. If it hits, you get to roll again and after that any 10s automatically grant you Righteous Fury.

So, the Death Cultist automatically triggers RF with any roll of 10, no need to roll a new attack.

Okay, so the blood cult assassin would then follow the 2nd edition WFRP rules for Ulric's Fury (which is what my DH group seems to have been using instead) and instead of rolling the 'to hit' test for further damage, you just go straight to the damage?

And then by comparison the Cult of Death assassin would trigger RF on the 7+, but would still have to roll WS/BS to add the additional damage?

Exactly !

Awesome. Thanks folks!