I actually started arguing this point with Origins and kept badgering them about it. If you want to do blood magic "right" given how it's portrayed in the game WORLD then taking the Blood Mage specialty should radically change your entire game. So you'd basically be making two games--the Blood Mage game and the Everyone Else game. This would put WAY too much mechanical emphasis on that ONE specialty. It would be the equivalent of having a game where you have classes called "everyman" and "titanus, destroyer of worlds". You just can't build a balanced game that way.
I am HOPING that they did it so that you can choose to use Blood Magic as a DECISION POINT via conversation and, yes, it has big consequences. THAT would be good design, particularly if they do those decision points in such a way that other classes/races/whatever have similarly large decision breakdowns. This would enable them to keep Blood Magic as something the PC has access to without the huge specialty imbalance. And without just making NPC's oblivious.
Given the focus on Specialties, the ability to only choose one and for their to be specific quests/sections tied to them, I thought this would have been great for Blood Mages.
But, then again, as you said - Blood Magic is a backdoor to untold power, manipulation and abilities in the lore. Having it be anything less than that in the gameplay and you run the risk of making any negative reactions to Blood Magic by NPCs as overcorrection. But, then again, if Blood Magic was that powerful, then there would be a huge reason to always choose that as your Mage spec (and, indeed, choose Mage as the desired class every time), so there is that.
But, that being said... having a Somniari in your party is equally as bad, simply because a somniari is said to be able to be one of the rarest types of Mage and able to manipulate the entire world with their will and thoughts alone. Not to mention are apparently able to complete other "impossible" acts in the lore, like teleportation. So I have a feeling his powers will be exceptionally diminished from what we have been presented in the lore.
Such is life. DA can never have Mages be as powerful (or dangerous) as they are presented in the lore, simply because there would either be INSANELY overpowered at any level, or they would run the risk of being possessed, hunted down by Templars or any number of other death scenarios.