It really doesn't sound like a good thing. It says here that it was outlawed after the ritual needed to awaken the ability left people dead because the new blood mages went berserk on the other participants.

[She closes the book, but hesitates to put it back on the shelf yet. Is this important information that she ought to take notes about?]

...

Nothing like that has happened in recent memory, right? Not to our knowledge, at least.