And this is somehow your evidence that Blight isn't just as harmful to Forsaken? It was designed to kill even less biologically active undead (like skeletons) and fully biologically active things (living). How could it possibly have a weaker effect on Forsaken?
EDIT: Might I also remind you that the Forsaken tried to make Blight turn things into undead so they could reproduce. They could never get it to work, so they settled on what it is now: a killing weapon that doesn't infect with undeath (aka corruption).