A good way to level up blood magic rather aggressively would be going to a drauger swamp spawn point and covering up the spawners so that your 2 skeletons can constantly fight draugers and the odd skeleton that pops up. Make sure to use the protection staff to keep them alive (though exp is only given if the player loses the bubble - not the skeletons).

I will agree that the skeletons are rather lack luster. I wish the devs added like 2 skeleton variations for enemies and the summons that have fragments of armor. Both mountains and plains could use such enemies when they spawn during the night or in the odd mountain ruin.

Make sure you have a rest spot though if you are farming drauger spawns otherwise your exp gain will be bad.