All those bugs people write about make me almost happy I don't have the game yet...
Back to Blackwall
I don't know what happens with the Wardens in DAI (haven't spoiled myself that yet), or if we know what happens to our Warden from DAO, but... Assuming the option with Blackwall joining the Wardens after DAI, I've been wondering how a meeting between him and Warden Commander Cousland would go, considering the past... Wardens being transferred from one country to another isn't something unheard of, after all.
*goes back into lurking mode*
The noble who hired him could be equated to Howe. It would be similar if Howe's captain was told that the castle would be empty except for the father, say if all the family had left for a summer home or something. Then he leads the attack, and finds the house full of people, including children. His men already fighting, he doesn't do anything to stop it.
My Cousland forgave Nate, who was part of the family. Her bloodlust was slated when she killed Howe. He's the one she blamed, he's the one that wanted it done. She would be wary, but after everything she's gone through I think she would understand that he's been trying to atone, and not give him too much grief. It would take a long time for them to be friends, since she will never quite be over finding her nephew slain, but were they to work closely with one another, I think they would eventually get past it.
My other wardens (I have so many, at least one from each background) would all react a little differently, but I can actually see him and my Brosca really hitting it off. She was quite full of herself and skills for a while, and did several things for money that she wasn't too proud of -- she wouldn't get his idealizing the Wardens (she never has), but she would get how he ended up in such a bad place and wouldn't hold his past against him at all. They sure would bad mouth nobility together, that's for sure!
It's kind of fun to think of him with each of my Wardens. 