Sorry, I got sloppy and started using the two terms interchangeably... my point still got across, I hope. Normal, mana-based spellcasting cannot be considered to be more risky than blood-based spellcasting since it is used by all Circle mages yet does not result in demons swarming into the mortal realm on a regular basis, if ever. The idea that blood magic is somehow safer because it supposedly doesn't draw power directly from the Fade is conjecture at best. We even have an example of blood magic being used specifically to draw power from a demon in DA2 when Merrill uses blood to open the barrier to Sundermount. Although she disagrees on the exact nature of the creature, she admits to calling one and used blood magic to do so, thus blood magic does not bypass the Fade entirely and is clearly shown to be capable of drawing the attention of spirits and demons.
Its not conjecture - blood magic is not tied to the fade. It is the only magic tied to the physical world. The DA2 specialization states that.
"[...] but this specialization isn't limited to madmen and monsters. Many see it as the only form of magic that is truly free, because its tied to the physical, not favors to spirits or demons." - http://dragonage.wik...(Dragon_Age_II)
Casting from mana is more risky than casting with blood. It isn't frequency of possession that determines it being more risky - its mechanics. There is always a chance that a mage drawing on the fade to cast can come to the attention of demons. There is zero chance of that happening when a mage casts with blood.
Merrill called on and summoned a demon, yes, and blood magic is also commonly used to summon demons, but not all blood magic, as the entry above notes, requires that. It specifically says that blood magic is considered a free school of magic because it is not tied to the fade or to the favor of spirits and demons. Merrill's use of blood magic, a more specialized use, would not fall under this general statement, but that does not mean that the statement is never true.