Mages who experiment with blood magic, so basically those who would be using a large quantity of it such as mass sacrifices or things of a similar nature. It doesn't say those who use blood magic, it says those actively using a large amount of power via blood magic. I fully admit, causing a lot of pain and death can thin the veil. Does that mean a blood mage sitting at a bar doing nothing and minding their own business will suddenly turn into a possessed killing machine? No.
And I do believe that note was in relation to the letter written in the section next to it, which mentions 2 individuals who used blood magic in piratical applications and didn't become an abomination, and two who used experimented with blood magic to do different and untested things such as to enter the veil and one who tried to become a perfect human being and both resulted in horrible abominations that way.
Using and experimenting are two different things. If our characters get the option to use our abilties to do some kind of untested ritual involving a lot of people dying to fuel it, then fine, abomination me and give me a game over screen or possession test. Fine, that would have been my choice and I should live with the consequence of that choice being a logical outshoot of what happened. But don't just spam me with initiative checks just becuase I decided to walk a few steps like this was some final fantasy game.