I contest the claim that new lore automatically overrules the old. If there are contradictions, then it's the writers' task to make things clear. However, that only applies if the lore is presented out-of-world. Almost all sources of Thedas lore are presented in form of in-world accounts, and refer to single events and are written from different viewpoints, so naturally there can be discrepancies.
We can infer few, if any, hard laws about blood magic. We can infer a lot possibilities. Most spells *can* be powered by life essence in form of blood. Blood magic *can* be learned from demons. It *can* also be learned from books. Blood magic *can* make you hear demons better. Blood magic makes it possible to cast *some* spells without touching the Fade.
There are a lot of unanswered questions. For instance, the only known way to cast mind-control spells uses a blood magic spell. If blood magic is often insufficiently understood as Last Flight hints, that raises the question if there is any theoretical consideration that makes mind control always work that way, or if there are alternatives which may just require more lyrium than is available to most mages at any time. Blood magic makes it possible to invade others' dreams. A Dreamer can do the same but does not require any blood. And so on...