I have since calmed down and have a bit more compassion when it comes to Blackwall now, because otherwise I would be somewhat of a hypocrite, but I still can't pinpoint why Solas was inherently "good" in my book and Blackwall "bad".
I did the same thing with Solas and Blackwall. I'll happily admit that part of it was superficial. It's a video game, a form of entertainment, and if I find one character more attractive than the other I have a tendency to cut them more slack 
But I think for me, what exacerbated the problem was the nature of the reveal, and the role it played in the narrative. Solas's came at the very end, as a stinger not only for the character, but the story as a whole. And looking back on it, there's that point in his final romance scene where he almost tells the truth, not because he has to but because he wants to. Because he cares so much, and feels that Lavellan deserves the truth from him.
With Blackwall, if you never get his personal quest, he never comes clean or even comes close to telling quizzy the truth. And I know it's a little metagamey, but between that and the way his quest is structured with the guy who nearly gets executed, I couldn't help but feel that Blackwall only told the truth because circumstance forced him to. It doesn't help that his weird and distressing animal cruelty story came RIGHT BEFORE the quest, so I was already thinking, "WTF is wrong with you dude."
I dunno. I honestly think it comes down to implementation, and Solas's worked for me while Blackwall's fell a bit short.