I'm not about to advocate for his (or anyone's) romance since I'm a repulsed aroace, but as far as Blackwall the character goes, I don't think he's as bad as many other present or past companions, "LI" or not. Even as a liar playing and using his bedmate, he doesn't hold a candle to Iron Bull or the ever-popular Solas. The main difference between him and most of these other characters is that for the most part he makes no excuses for himself. He is guilty as hell and he loathes himself for it -- that makes him vulnerable and attracts player hostility in a way that even many of the gleefully murderous and/or evil characters don't. Because being unapologetic and proud of one's horrible actions is "cool", while being sincerely guilt-ridden and remorseful is "weak". We are not invited or allowed to judge other characters as we can him, so he gets a lot of hate while other characters get admiration and excuses. Leliana, for example, is never called out for anything she did and does, instead we're meant to see her as maybe a slightly whacky crazy-nug-lady but ultimately as an admirable and charismatic reformer.
My main beef with Blackwall's arc is that there is no possibility for a slow rebuilding of trust and friendship. He is guilty, no bones about it, and because he's very human his quest for redemption is as flawed as it is sincere, potentially causing trouble for the Inquisition along the way. Bouncing back from all that would take most people quite some time ... but we don't get that. One moment he publicly insults us, the next moment we're instant buds again. The only lasting consequence is the broken bond between him and Cassandra, which I suspect most people will never see since running with more than one tanky warrior type (if that) doesn't seem to be very popular.