The dwarves don't know where the blight came from. They just know 15 years or so after the 1st blight started darkspawn started invading the deep roads.
That is blatantly false. The blight didn't officially start until after they lost most of their empire.
I am replaying Origins as a dwarf noble, and I've carefully read every single bit of lore I gathered in the origin, especially those concerning darkspawn. The dwarves outright dismiss the idea of darkspawn coming from the surface or the Fade since the darkspawn started attacking them from below even their underground empire. History of Orzammar chapter 1 talks about missing travelers, or small isolated attacks, so they increased patrols and called it good.
Chapter 2 discusses how large the darkspawn army was, and how they were completely overwhelmed, losing thaig after thaig. The dwarven codex on darkspawn and blights talks about how an Aeducan scout thought they were winning the war when they came across the main body of the darkspawn; he saw the archdemon before they started pushing towards the surface. Aeducan, Shield of Orzammar discusses Paragon Aeducan and how he united all the warrior caste, bypassed the nobles and the assembly entirely, and pushed back the darkspawn from Orzammar, and how his insubordination earned him Paragon status, and he still considered it a defeat that he couldn't save more and was forced to seal off most of the Deep Roads.
The dwarves however completely and utterly dismiss any notion of the darkspawn originating on the surface since all their records going back thousands of years say that the darkspawn came from below even their lowest thaigs.
At present, there is only one place known in the games and lore that is below even the Deep Roads and the dwarven empire. The Primeval Thaig.
Is it concrete proof? No. But it is enough to make it clear that there is far more to the origin of darkspawn than the Chant of Light, or even Corypheus would have us believe. There are too many unanswered questions.
Like, if the Corypheus and the other magisters truly were the first darkspawn, how on earth would they know how to make broodmothers? How would they know where to find the Old Gods since, as Awakened Darkspawn like the Architect and as Corypheus confirms in Legacy, they can't hear the song. Or how they somehow managed to sneak into the deep roads at the height of the dwarven empire, go through patrolled tunnels, and build an army without the dwarves ever noticing? Also, since Corypheus survived the ages, sealed by the Grey Wardens, what happened to the other magisters? Were they killed in the blight, were they sealed as well, were they captured by the dwarves, or something else?