I think the talk with Krem is more hypothetical. If you could change yourself with magic would you do it? Rather than automatically assuming it is possible.
I also don't think the blood magic was changing Dorian's underlying sexuality but simply controlling his mind to accept his situation. Hence the possibility that his sub-conscious would still be fighting the compulsion and the conflict would break his mind, rendering him a drooling vegetable. Of course I could be wrong about this but the way I understand the blood magic control is that it has to be maintained by magic and that if it wears off then theoretically the person would return to their previous state of mind. However, usually the mind has been so badly damaged that they die. In DA2 control of the mind seemed to be linked with actions of a demon, so the blood magic summons a demon who then exercises control of the person on your behalf. May be all these ways are true; after all we have never really gone into the mechanics of blood magic in depth like they would in Tevinter.
The Qun can "fix" people's minds without the aid of magic, simply by using standard mind altering techniques, so I suppose blood magic could also be used to alter their mind permanently as an alternative.