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Ironhawk and Uninspired are right (and much more polite). Reducing to the essentials:

* Gold increased in dom3
* Gems didn't increase in dom3
* Blood hunting equation: gold + time + unrest = slaves

Since gold increased and slaves come from gold, slaves are easier to get relative to other types of gems (but harder to get relative to gold because of unrest).

Thus blood magic got stronger in dom3, not weaker, because it should be compared to other forms of magic - not to recruitable troops. Of course blood got weaker compared to recruitable troops, everything did, no one disputes that.