I thought I read somewhere that dwarves said the Darkspawn have been around before the Maker, but like GW call OG Archdemon's even when their not, perhaps "Darkspawn" is the only word they know for it, but they were something else before that? We also know that talking The Scaled Ones who dressed like Tevinter Magisters performed blood rituals in the Deep Roads too.
The dwarves say the darkspawn came from beneath, and that discredits the whole ancient magister being cast out of the heavens that the Chantry preaches. The dwarves believe that in general, although Cory seems to prove their dismissal of the Chantry's tale to be wrong, but darkspawn did appear deep in the Earth before reaching the surface. Dwarves had no idea what the darkspawn were until they were being overwhelmed by them.
Darkspawn are beings born of the blight and wholly of the blight, they are not infected things like broodmothers, Archdemons, or Grey Wardens, which are all ghouls in some form. Supposedly, the Ancient Magisters (like Cory), were so transformed by whatever happened to them, that they became of the blight, completely, they are not infected humans, they are the first darkspawn. This is how Cory can body jump, he rematerializes using the blight and tears himself out of the victim, no soul clashing occurs.
The Scaled Ones sound pretty cool, and they battled the dwarves sometime between the rise of Tevinter and the Fall of Arlathan, so they have no connection to the darkspawn, but they may have a connection with reavers and possibly elves, since they were apparently in shock of one of their own being killed. They do blood rituals, and darkspawn don't. Darkspawn use blight magic, as highlighted in The Calling novel.
Nonetheless, different sources do hint to the blight being far older than the Second Sin.