It's likely a balancing issue in terms of class specializations. Really, being a Blood Mage is the only class specialization that could have your character be openly discriminated - even hated - against. While it would be neat and more "realistic" if other people actually acknowledged that you're a Blood Mage, they probably thought it'd also be "unfair". That, and depending on how they did it they might have to re-write entire chunks of the story to accommodate your decision to be a Blood Mage, and I'd imagine that'd be a lot of extra work during the development process. Because yeah, basically as soon as it becomes known that you're a Blood Mage...Grey Warden or no, you're going to be hunted by every authority figure in Fereldin. Quite a different story to the one about a hero uniting the armies of the land to combat The Blight.