Well, KainD, you should start with finding the research journals of Avernus and First Enchanter Remille (the First Enchanter of Fereldan before Irving), those would be a good start.
Really, Tevinter should hunt for that information that Remille had, he was able to create enchantments that manipulated the taint, either speeding it up or resisting it. He was even able to use blight magic by making that cold, dark, shadowy spell that consumed his victims, and he wasn't even a ghoul as far as I know/remember. Does one even need to be tainted to use this magic? Does the taint need to have physical manifestations in Thedas to have such magic exist?
While the darkspawn are "new," maybe the taint did not originate with the first darkspawn, maybe it was always deep in the world and the first darkspawn, the ancient magisters, were just cast into it from the Fade to be consumed by it? Maybe the taint as we know it is a corruption of something more primordial deep in the world, like how red lyrium seems to be a corruption of blue lyrium?
The Inquisition will hopefully shed some more light on this after their assault on the Grey Wardens as was seen in a trailer.