I can understand Blackwall's reasoning against Loghain. Loghain's reasoning for keeping Fereldan troops out was sound, if a bit screwy, but it's not like he did it for shits and giggles. He wanted to show that Ferelden could stand on its own, that it could rally itself together after Cailan basically killed himself and not fall to petty infighting and a civil war which would weaken them (as that was the precedent Orlais used for conquering nations in the past. "Saving" them and never leaving).
Ironically, his strong opposition and ******-poor political knowledge antagonized the Bannorn, causing people like Teagan to refuse to ally with him against an invasion and put honor before reason, with the Bannorn striking the first blow.
And from his point of view, the Wardens were responsible for killing the king. Not through a direct act of sticking a blade in his gut, but by duplicity and manipulating the man's love of the Wardens into goading him to bank on Wardens alone being enough. Couple that with other failings at Ostagar and it seemed like the Wardens had returned to being Orlesian agents, willing to sacrifice Loghain and Cailan if it meant Orlais could come in and save the day with their martial might.
And to be honest, when you have a bastard Theirin in your grasp who was raised by an Orlesian (Isolde) and a traditionalist (Eamon) then things would look pretty suspect. I'd think Alistair was being used as a puppet myself to satisfy nationalist/traditionalist interests while at the same time allowing Orlais to call the shots, particularly if it meant the dethroning -- and as such things go, execution -- of the person who has been the brains behind the nation since Maric's death (Anora).
Was he right? Yes and no. Orlais did have designs on Ferelden, it seems, but the Wardens weren't a part of it. But I can see how Blackwall would have problems with it all.
Though I do wish Bioware didn't adhere religiously to this "Loghain was a BAD BAD MAN" mentality, even with the whole "history is written by the victors" truth.
To me he'll always be Kain as well and Judge Magister Zargabaath.
(course he also voiced a senator in the same game as Zargabaath (FFXII)).