Does this mean that the Blight was not what made archdemons immortal and Corypheus but something else? Up until now we thought it was the Blight that caused archdemons and beings like the Architect and Corypheus to be hard to kill. But if that 'effective immortality' is the same that Solas and Flemeth were doing, then it can't be the Blight since Solas and Flemeth were obviously not affected by it.
The Blight doesn't make people immortal .
Although the truth is the Blight is a complicated matter at this point.
We have the Blight infecting Red Lyrium , it feeds on the living and grows some more , it doesn't echo of the Old God song , Behemoth and Red templar didn't dig for an Archdemon.But it also sings.
As for immortality with Blighted Lyrium , we don't know .Not enough time to study red templars/Meredith.
The Blight from the Golden City has its own weird effect:
There is the 7 magisters of the Golden City , they kept their soul , they are immortal , they aren't under the compulsion to seek the Song of the Old Gods.They were tainted in the Golden City , so they are special snowflakes.
The Blight in the Golden City doesn't work like on Thedas.There's never been a city forever turning black after a blight on Thedas.
Then there is darkspawn, they are born out of the Blight , they are immortal (sustained by the Taint)but bound by the Song , they may be soulless.
Then there is the Archdemon , they show the same powers as Corypheus , able to body jump , able to use the Blight to create a hive mind and mind control.Except they are , it seems , the source of the Calling.
The main difference is the Archdemon can't mindcontrol wardens unaffected by the Calling , they can't override the warden's will and use their bodies for a new suit.
Then there is ghouls , animal or human , they simply become violent , they start rotting , hearing the song and they'll die after a while.
So ghouls prove at least if you're the average Joe on Thedas , the Blight isn't a garantie for immortality.