Sacred Haven of Debauchery

But there, it was stated explicitly.

"Hey, Hawke! Look what I've done here! By the way, Orsino provided me with shelter and also sent me books on necromancy, yes, yes!"

Do you know where the knowledge of blood magic came from?

Do you know that most legends were created by the Church, and it interprets history in its own favor? (read - to the detriment of Tevinter and mages).

And how do most mages acquire it?

If you believe Anders in DA2, most mages accidentally cut themselves on something and feel the energy. The rest, I suppose, study forbidden tomes, like Jowan. Only a few make deals with demons.