[rdonj]

The random number generator isn't necessarily going to produce them at a rate of 1 in 3, sometimes it will get more, sometimes it will get less. The more you make the closer you're likely going to get to 1/3 of them having blood. If it says they have a 33% chance for blood magic, they have it.

I have no experience trying to mod percentages of holy, the manual would make it seem like it would work but I could see it not working. My suggestion would be to make a unit with an 80%+ chance for extra holy magic and try making a few in a game.