First of all the Necromancer's Blood is Power passive does not seem to proc the 20% cdr when cumulatively losing 100% health as it states it should. Second it seems to be a poorly designed cdr passive even if it worked as advertised. As it stands it's just an inferior version of the evocation passive for wizards. If there is going to be a gimmick involved, then the resulting cdr granted should be higher than one granted w/o a gimmick.
On another Note I really think Blizzard is missing an opportunity to make this passive something interesting and unique in the game. My suggestion would be to redesign the passive so that when you cumulatively lose (some amount of) health it reduces all cd's by one sec. This way it will be possible to continuously benefit from the passive in a way similar to the grave injustice passive for Witch Doctors, but not in a way that is game breaking.