I see in 'that thread', and in others, that one factor in the Blackwall-hate appears to stem from the fact that he leaves after sleeping with the Inquisitor and chooses not to tell her the truth before sex, and before he leaves Skyhold.
The impression I got, and my own personal headcanon, was that Blackwall had already decided to leave before the tavern/barn scene, and wants to spend the last evening in Skyhold with the woman he loves, the woman he credits with making him finally face the demons of the past. I definitely don't believe he made that decision after sleeping with her, "Well, I've had her in barn, I've got what I wanted - now I guess I'll leave". He thought he was going to his execution - you can't blame him for wanting his last memory of his lady-love to be a positive one, not her all upset and betrayed, but curled up cosy in bed, happily dreaming after they had made love.
Blackwall isn't everyone's cup of tea at all - he seems to have spawned several threads, containing at times angry debate over his actions and motivations. And people say the character is boring?! Isn't all this debate testament to Bioware's skill as developers and story writers, and their ability to stir our emotions and celebrate diversity - in this and other Bioware games, there's truly something for everyone. Want a sweet, sugary, Disney-prince romance? Try Cullen! Would you like an LI who isn't always serious, something with more humour? There's Sera! Feeling a bit kinky? Here's Iron Bull! Perhaps you'd like a broody beared older man with a bit of a dark past, a darker romance arc but one ultimately about redemption? Let me present Blackwall.
Bless you Bioware, for doing what you do to us.
(small disclaimer: I haven't done all those romances - they may be horrible summaries
)