Some comments might get someone confused, so I'll add my few words in too.
Blood slaves are the Blood magic counterpart for the gems of other magics. You need BLOOD, not just colourful minerals.
Blood Sacrifices have nothing to do with magic: they are religious rites. Each sacrificed slave (more can be sacrificed per turn, propably limited by the priest level of the sacrificer) forces one "temple check", that is, each sacrificed slave spreads the dominion just as a temple in that province would.
In short, Blood Sacrifice increases you dominion.
Re: Blood Slave/magic Questions
Threaded Mode