I just wrote this up for someone in my inbox: it’s the most thorough description of What We Know About The Blight that I’ve done in a while, so here are the important bits without any of the personal, private-reply stuff:
1. The Blight acts exactly like magic.
- During Broken Circle, certain party members (Morrigan and... Alistair? I think?) will comment that the squishy organic ooze in the higher levels looks remarkably like the Blight.
- The Origins codex entry for lyrium tells us that ‘Mages [who take too much lyrium] have additionally been
known to suffer physical mutation: The magister lords of the Tevinter Imperium were widely reputed to have been so affected by their years of
lyrium use that they could not be recognized by their own kin, nor even as creatures that had once been human‘.
- The Taint can be used to
cast spells, which is how genlock emissaries exist and also why Corypheus gets twice as much power from red lyrium as from normal lyrium.
- With normal magic, you get blue lyrium. With Taint magic, you get... red lyrium.
2. It’s probably somehow connected to the Void.
- ‘Yet even a god should not linger there, and each time she entered the Void, Andruil suffered longer and longer periods of madness after returning. Andruil put on armor made of the Void, and all forgot her true face. She made weapons of darkness, and plague ate her lands.’ (Here)
- ‘[...] the Empty Ones grew to believe that the Blight was to be the tool by which the Maker would end all of creation. They preached that it came from the Void, a place of nothing,’ (Here) (That’d better not be a hint that Solas is going to start another Blight in DA4...)
3. The cure for it is probably having been possessed by a spirit.
Note the wording, because it’s important: the cure is not being possessed, but having been so. Anders still has the Taint because Justice is still with him*, but Fiona - who became possessed in The Calling and was saved in the same way as Connor can be - does not. This would also explain why Seekers seem to be immune to it; they’re possessed too, but only very briefly. And Leliana’s Blight resistance is probably from a similar source: I think her ‘vision’ may have involved being temporarily possessed. (A reminder here, as well, that if you get the Circle to cure Connor then Irving makes a vague reference to subjects of the cure being somehow ‘forever changed’. Considering the Seekers have undergone a very similar experience, and also that Connor can be found in Redcliffe Castle in the Dark Future when Alexius is experimenting on Blight-resistant people, I think this may be what Irving is referring to.)
Avvar mages are probably all immune to the Taint, as are cured Tranquil (see: Seekers).
*Possibly also because Justice was previously in a Warden body, and may have picked up the Taint and brought it with him into Anders?
Extrapolating from that, and from what Karl says about Anders ‘bringing the Fade into the world’, I think the cure might actually be being touched by the Fade, rather than a spirit per se. If Solas destroys the Veil, it’s possible that he would cure every Grey Warden in Thedas as a side-effect...
since it’s now looking very unlikely that the Veil exists outside of Thedas, the Hero of Ferelden may have cured themself simply by leaving the continent.
4. So to sum up...
If we take normal magic to be ‘Fade magic’, i.e. magic that comes from the
Fade, then the Blight/darkspawn magic might be ‘Void magic’, i.e. magic that comes from the Void. This doesn’t explain how it got into the Black City (my guess is eluvians) or why the Blights are a thing (my guess is Veil + something to do with dragons: ‘the blood of dragons is the blood of the world’, etc), or indeed what the heck darkspawn even are, but I believe this is a solid foundation in what it is. And from there, eventually, we might get to the rest of the answers.