Hi,

A newbie just playing a game and was wondering about how "Can't be killed" interacts with "Can't be saved" effects. For example, Wildfire Assault interacting with Power of Blood, and a player having 3 Noble characters and 3 non-Noble characters.

Can I select the 3 non-Noble characters as the ones to keep, and the 3 Noble characters still survive the Wildfire Assault as Power of Blood is active and says they can't be killed? Is a "can't be killed" effect still considered a "save"?

Yes. It's one of those things that seems too good to be true when you first realize it. For more explanation see this thread, this thread, this thread, or this thread.