Uh, one more random thought before I get back to grinding through a weeks' worth of fic, theory, and tears:
One of the things that kept me from adopting the theory that the blight was directed by an entity whose goal was assimilation/domination of sentient life on Thedas was that blights ever end. If the song/hive mind was directed by a non-Archdemon Something (something awake and aware enough to have called Tevinter priests and warped the Andrastean religion) why would attacks on the surface be limited to times when a blighted Old God/Archdemon was active?
I can think of two possibilities, and they're not necessarily mutually exclusive.
1) For the Big Bad to be released, the Dwarves have to go first. My theories on 'why' are still half-baked. In a way, I think the Dwarvish race might function a bit like an immune system for the Stone. Carving away the bad parts, beating back the physical infection, with Kal'Shirok dwarves possibly working like T-cells, removing the blight from the Earth's blood directly. I'm not sure how much I like the image, but it kind of fits. In that mindset, my brain does weird twitchy stuff when I read things like this:
"In this place we prepare to hunt the pillars of the Earth. Their workers scurry, witless, soulless. This death will be a mercy. We will make the earth blossom with their passing."
The "witless, soulless" part is obviously meant to trigger our "gotta be darkspawn" detector, but I think that might be a misdirect. Consider instead:
Pillars of the Earth = Titans (possible keys to Big Bad's imprisonment) Workers = Dwarves. Blossom = blight.
I think it's safe to assume the original Dwarves- whatever they were before they lost/forgot their original purpose- were much different from what they are now. Further assume this progression is a trajectory- that they're growing further and further away from being Servants of the Stone until you have someone like Varric, a surfacer who can't hear the Stone at all and is, in a meta sense at least, practically a Dwarven mage. (In so far as being a writer is a bit like dreaming while awake. Crafting and shaping worlds that aren't, but could be, etc, etc. The fade-y connections are all over the place in his dialog with Cole.)
Anyway! Moving backward in time and downward in space from something like Varric, you lose more of the Dwarf-as-individual, and gain more Dwarf-as-Stone-song-driven weapon/tool. Combine this with Dagna's crazy experience after the resolution of "The Archanist and the Fade", and you have the Dwarvish race as a networked hive mind, connected by blue lyrium. Built, perhaps, to counter the blighted red one and maintain the integrity of the Big Bad's prison.
Ugh. Totally weak theory. I don't like it yet, but there you have it.
2) Surfacers think of blights as Darkspawn attacks... but what if they aren't? Given what we think we know about the Big Bad, when there's no competing Song from an Archdemon, wouldn't it make more sense for the Darkspawn horde to be digging down? It's possible the Old Gods, maddened as they are with blight and pain, have just enough them left when they wake for the thought "UP", dragging what they can control to the surface, away from the Big Bad's objectives. So... not an attack. Just a temporary setback, a roadbump slowing down the Big Bad's plan to free himself to rule over an enslaved and blighted world.
If so, nice job again there, Wardens. =w=