Apparently we won't be allowed to do blood magic for that very reason, because it's supposed to be "evil". Of course, that doesn't mean the Mage wielding it is bad, and in the case of Merrill she resulted to it for the sake of her clan, there were good intentions behind it.
I think blood magic is tough to implement in general, and there have been a lot of (fair) complaints that the lore and gameplay don't match up on this matter.
Blood magic is supposed to be completely OP. Blood magic is supposed to make you more vulnerable to possession. Blood magic is naturally going to get a violent response from party members and NPC's unless you're able to use it to manipulate their minds so that they don't notice. Blood is in limited supply, and when you've used so much of your own, you'll need to take it from party members or slaves or bystanders (which would probably get some/most/all companions hacked off too, naturally) 
It's supposed to cause a whole bunch of personal and moral dilemmas which would kind of necessitate that a protagonist be completely blood mage and no other classes and specializations allowed in order for the writers to do it justice. :/
Blood magic also weakens the Veil further, which is kind of counter-intuitive to what the Inquisition is doing.
I'd be surprised if Dorian doesn't use it though, or didn't at one point. One doesn't become a magister in Tevinter without blood magic.
Cole has definitely used it in the past. It'll come up -- and not just because of eeeeeevil villains using it. 