Everyone knows that there are no free mages under the Qun. The Ben-Hassrath know that there's nothing more useful than a fact that 'everyone knows'.

Anders: So, there must be mages in Tevinter that don't use blood magic. Fenris: Of course. There are slaves. The magisters do not hesitate to collar their own kind. --- Or, the one where Danarius finds out about the blood magic ritual his long time enemy Halward Pavus is planning and things go even worse for Dorian.