I just don't get that part. Do you honestly expect a Gw to say:
"Ah, the Wardens. See, the way it works is that we taint ourselves with darkspawn blood. Can't blight what's already blighted! Except we use a complicated blood magic ritual to slow it down. Don't tell the Chantry about all that blood magic! It comes from the Imperium you see. Oh, we eventually go insane and turn into monstrous ghouls but that takes years. It takes quite a while before you go insane and your flesh falls off in chunks. We know if there's a blight because we can feel the Archdemon literally touch our minds. Our endgame is to destroy our soul along with it. If there isn't a blight we go off and commit suicide in the Deep Roads. Did I leave anything out?"
For me Blackwall sounded like he genuinely struggled to answer the questions for which he just didn't know the answers. He sounded ignorant, not evasive. And that's why it was so suspicious.
The minute he claimed to be in Ferelden fighting darkspawn during the blight, my alarm bells went off. He acts like a heroic and chivalrous grey warden who fights for justice. Why would he go on gallivanting alone in the outskirts of Ferelden when my Warden tries to save the world almost on her own? At that point, me the player was pretty sure Blackwall murdered real Blackwall and assumed his identity. Partially incorrect, but my suspicions were well-founded.
My inquisitor (who has less super secret information on wardens than me) got suspicious after taking Blackwall along to meet with Loghain. It was damn strange and also arrogant how BW just claims to be this extraordinarily strong-willed person who doesn't have to worry about the calling. Loghain is one of the people who defeated the blight! Why would Blackwall be so much superior than all the other wardens in the area, including Loghain?
The thing that was the most suspicious to me though, was Blackwall's general attitude. He idolized the wardens and thought them as saints. His attitude was closer to that of a rookie warden who doesn't have a clue how far that organization is willing to go to achieve their goals. If Blackwall really was a senior warden, he should have been more like Duncan or Clarel, not have a childish hero-worshipping syndrome like Alistair did.