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Blood magic is all about the summons. The combat spells of blood, while decent, have a lot of overhead in the form of the blood slaves, who are often killed by enemy action (sometimes without even trying), and even if not killed, get in the way.

Blood summons, however, are extremely strong. Ice Devils, Arch Devils, and Demon Lords are very powerful units who are better SCs than many pretenders, and often require minimal equipment to do their job. And, they are unique. All the blood nations need to race to get the powerful blood summons, and the limiting factor for this is usually blood slave income, rather than research. Mictlan's enormous bloodhunting capacity is a big advantage here.

Mictlan dominion, while micromanagement-intensive, has a lot of advantages. You can "aim" your dominion very precisely with Mictlan. If you take a hostile dominion, such as drain scale, you can simply avoid putting your dominion in certain provinces, for instance, carving out a low dominion area to do your research in. You can also push your dominion, suddenly and effectively, into enemy territories. This makes vampires (another blood summon) extremely effective. The ability to move your dominion (combined with low cost temples) also allows you to get away with slightly lower base dominion than you otherwise could. Mictlan is also highly resistant to being dominion-killed.

Send Horror is a powerful artillery spell, with an extremely low cost for what you get. It's not quite as popular as Ghost Riders, but this may be due to the difficulty of casting it. Horrors are extremely powerful against some types of armies and extremly weak against others. If you saw no effect at all, you must have done something wrong, as you do get to see the battle.

The in-combat Call Horror spells aren't quite as good, because whoever conjures the horror up is likely to be killed by it.