[Baalz]

Methel is correct, which is why blood hunting with scouts raises unrest...even though ten of them blood hunting often find nothing.

There are several checks which happen (unrest, pop level), but IIRC the one with regards to blood magic level is 40% X blood level chance of success, it also effects the amount of blood slaves you get (only +1 per level though). Generally 3 level one mages with dousing rods is the best way to go for cost/benefit, though that's not an option for everybody.