Lets say my opponent has 6 characters, the Blackfish, Catelyn Stark and 4 non-unique characters. I play Wildfire assault as my plot :When revealed, each player chooses up to 3 of his or her characters. All characters not chosen are killed (cannot be saved). He plays Power of Blood: You may have up to 2 copies of The Power of Blood in your plot deck. Noble characters cannot be killed. To resolve my plot he chooses 3 of his non-unique characters and kills the last non-unique. Was this performed correctly? Does the power of blood make the noble characters illegal targets for wildfire assault? Or did my opponent pull a fast one on me?

Nevermind, I answered my own question as soon as I posted this. Thanks anyway…

Yea, since Wildfire doesn't "target" those it kills it is a legal play.  The "cannot be killed" of the plot is not considered a "save" so the "cannot be saved" effect from Wildfire doesn't come into play for those nobles.

I thought that it depends who resolves his plot first,

when it would be the one with power of blood then I think It is as you sad SLOTGODFATHER, but when it would the one with wildfire assault  then I think the other player must  choose among all characters, because his plot didn't resolve yet and they can be still killed, or am I wrong?

Power of blood is not a 'when revealed' plot card. It is active from the moment you turn it over.

Underworld40k said:

so the initiative is only for choosing the first player, and that first player only choose the order in which are the "when revealed" plot cards resolving, all others plot cards are immediately in effect?

Pretty much. All constant effects on plots (like Power of Blood) are active from the moment the plot is revealed. Some plots have passive effects that have other triggers than "when revealed" (The First Snows of Winter initiates after the first challenge of the round is initiated) or even a response (Outwit has a cancel response that is usable from the moment the plot is revealed). Threat from the North is an example of a plot with both a constant effect (the STR penalty) and a non-"when revealed" passive effect (the discard at 0).