Well it makes sense that you had to do it through the cultists considering the blood has to be ritually prepared and they are the only ones who know how to do it. Plus considering the fact its a form of blood magic and it makes you more violent its not exactly a "grey" spec. What I did was I was against people using the ashes sooo I sided with Kolgrim poisoned the ashes so people can no longer use them and then after I got the reaver gift I murdered kolgrim and his cultists. Took his horn and murdered the high dragon.
Then I pinned the poisoning on the cultists, the chantry did not believe him and he committed suicide the ashes cannot be misused and the cultists are gone and the dragon and the chantry now have a historical site that can be used. That's the outcome I wanted.
Win win for me if you can't handle that you have to deal with unsavoury people to get an unsavoury spec don't do it that's it. Its bad enough that people reloaded to "cheat" their decision through reloading. Necromancer, Reaver and Assassin are the "dark/grey" specs of the game. I suspect one of the many reasons bioware changed how we get specs and integrating it into the story is to prevent that.
Plus from the wiki says that the blood has to be ritually prepared the cultists are the only KNOWN people who know how to do it and ritually prepare the blood. The only other people are witches of the wild and how would you know to collect their blood to unlock the spec? If you kill kolgrim?
"Its is not easy to learn this gift as it comes from the ritually prepared blood of a dragon"