[Poopsi]

wounded (or even afflicted) blood slaves are usable, in my experience.

Now, blood magic... it IS extremely cumbersome... mostly because of the high fatigue rates, but also because it´s combat spells are expensive and not too good. The fatigue can be offset with a highly empowered spellcaster, or a communion (which I think is more practical to use with Astral mages as slaves, as otherwise you´ll be spending many slaves just to set it up).

If you do commune up your casters, make sure that you have the exact number of slaves you are going to use in your first five turns, and/or extra communion slaves. I had one communion with four slaves set to be reinvirgorated by a sabbath master while the other casted agony at the enemy (I sort of hope it to perform well with a death bless), which resulted in a wipeout of all the slaves when my caster decided to use his increased-due-to-communion blood level to cast the level 5 battle-wide life leeching spell, which has horribly high fatigue. My mages survived that, but in the following moments he casted mind burn four times, each one resulting in a dead turkey...