Maybe the song is always there. Maybe the Blight just destroys the part of the brain that prevents us from "hearing" it. That natural boundary in the brain that filters it out. I don't know if we always must go about how the song is put into a tainted one, maybe it is just that the taint removes defenses.
Like, let's say, air pollution. It is always there, all around us. But a few parts of our body filters it to an extend so that we can cope with it.
Again, not thought through, just a very inital thought.
I know nothing about it besides "not a Grey Warden anymore". So sticking to my theory of destroyed natural defences - what if her body simply regenerated those defences? Like healing of a wound? You cut your finger, it is destroyed and all the bad environmental influences affect you (simply put: it hurts, wind/water/dirt etc. hurts). But the body will regenerate the flesh and skin, it heals and the defence is back up. Maybe this happened to Fiona.
I understand the blight something like a fungus which can enter into parasite/symbiosis with the other organisms by infecting their blood, and being able to replicate.
This somehow fits me to:
- red lyrium carrying the blight -> crystal infected by a fungus
- blighted creatures being infected by the same disease, share the same goal (destroy the living, infect more to make the fungus replicate)
- I support the Grey Warden ritual of drinking darkspawn blood is a sort of "vaccination"
- the song is a way of how the fungus "communicates" with the living bodies it infected. The louder the song, the more the infected body is controlled
- Fiona's miraculous cure is somehow gaining organism self defence against the blight fungus and my instincts tells me this is somehow related to blood, particularly Alistair's blood.
I think the goal of the blight is to transform living organisms into somehow parts of a collective mind, though:
- the resistance to it depends of individual organism or the way how the blight was spread onto
- some (Corypheus?) know more about it's origins and may use it as a tool for their own purposes
I doubt though BW had something so literal in mind and wanted to create a "mystical" disease, more open for players' interpretation, not to flatten the story.
Disclaimer: I am completely lame when it comes to biology stuff, so... if anyone is educated in the topic, please treat my theory mildly
Happy to hear any feedback though.