The one change Dom2 really needs.....

[Yossar]

I never understood why blood sabbath cost so much (100) fatigue. That's the main reason I never used it. Even without requiring blood slaves, there's still going to be a lot of blood involved. The blood of the communion slaves. But it would be interesting for Mictlan to have fairly easy access to high level nature, astral, water, fire, and blood battlefield spells.

[Cohen]

Simply for default each gem cost for a battle spell == to 100 fatigue.

So Blood Magic isn't that used in battle magic because it costs not only slaves but a lot of fatigue.