Are devils worth 7 blood each?

[Cohen]

You're forgetting Vampire Lords and Fallen Angels, both good summons.

And Blood could be used to paradrop Hordes from Hell early and cause problems to your enemy (I underline early, later there will be many castles and stronger units), especially cutting out withdraw options to enemy armies that you're going to engage.

Not to count some good battle spells ... sadly you've to bring a lot of slaves with you to cast blood battle magic, and this is a shame IMO.