Humans may dwell here

A barrier would help.
Regarding blood magic: it's simply a very complex technique that requires extensive training and preparation, during which the mind undergoes significant and irreversible changes; something gets disrupted, and the mage loses the ability to use mana.

Alex, instead of trying to quickly expand the list of races that can use magic, wouldn't it be easier to remove the race restriction altogether?

No. The Chinese, Northerners, Knights, Imperials, and Lions cannot, and that's final.