The problem with this specific scenario you've set up is that, even with actual, factual, blight-infected flora and fauna, traditionally the people in the DA universe still wouldn't believe the Wardens.
You have to remember that Darkspawn are a constant threat in this universe. Even when there is no Blight happening, Darkspawn constantly are fighting the Dwarves down in the Deep Roads, and are also often making random incursions onto the surface-world as raids or slaughter parties. The good news is that you have some level of recourse.
You're with us now.
The Wardens have ancient documents that allow them to conscript people to their cause. They are only able to do this if there is an actual Blight, however. So while not everyone may buy it, if your Wardens go around invoking their ancient right to defend the land, some people may be intimidated into joining their cause.
Fine! We'll make our own defense team. With Blackjack. And Hookers.
So some nobles don't like your PC's. So what? The great thing about Ferelden is its diverse population and politics. Maybe the noble houses aren't lining up to get behind these Wardens, but the Chantry sees the danger and offers its assistance. Maybe the Dalish or the impoverished city Elves back your guys for political reasons as much as to defend themselves from a common enemy. The Dwarves certainly know the dangers of the Darkspawn. Perhaps they've seen the signs and are willing to help. Or hey, how about the towns of villagers who are nearest the Blight? They have ample enough reason to help. Or the mages. Or the templars.
In short, they don't necessarily need these nobles, unless for some reason Plot dictates that they do.
Finally, I'm sure there are things that will convince a crowd. An authoritative person brought to the source of the Blight to see for themself. Divination mages who can see the forces gathering (convincing them to expend the resources to cast the magic may be an adventure in and of itself!). Or, as you suggested in your answer, blighted flora/fauna may be enough to convince some people (though you seemed to think this a boring answer).
Just remember that Dragon Age is interesting not solely because of the threat that the Darkspawn represent, but because of the myriad of different attitudes and factions that inhabit the world, and their conflicting motivations and beliefs. It is as much a political intrigue setting as a dark fantasy.