Oh geez. There really should be more than 1 line of dialogue to activate a romance. Have any of you who have played other romances also noticed that the trigger is also one line of dialogue? I find that most of the flirt dialogues are too direct and ooc, and even though I'd like to romance a companion, I'd like to explore some of the other dialogue options besides being a total flirt. I mean, does the Quizzy have love on the brain the whole time? :-P If so, I'd better be very careful in my dialogue choices when I do the other romances.
Decisions decisions..... whether to play this through or to go back like 80 hours to the start of Skyhold. This is my canon run. Y u so hard to get, Blackwall? Everyone else in Thedas wants me to romance them (I have still have flirt options for most). 
I've only done IB and the flirts go right over his head a lot until he finally shows up in your room saying he's caught the hint, you wanna ride the Bull. Sera always seemed to be available, imo, even when I was fully committed to Bull (dragon necklace and all) and would tell me she wasn't gonna hook up while I was still with Bull.
For Blackwall, I didn't flirt with him until the "admire" heart icon appeared. That conversation was funny. And THEN it was easier to pick the "you're oddly charming" line because the ice was broken at that point.
And do you tend to replay your canon game? If so, just finish out the one you have and you can do it proper the next time. I had to do the same thing. See, I was avoiding spoilers on this game, but I had a couple of friends who didn't. They scoured the forums for every info they could get their hands on. And they were pretty good about not spoiling me, BUT. Somehow they got misinformation that if you let Varric keep the red lyrium idol in DA2, he was going to go crazy in DAI and you'd have to put him down or spare him as the blighted creature he turned into. They knew how much I loved Varric, that it'd break my heart to have to kill a fellow dwarf, so they told me. I changed my decision in the Keep, not happy about changing my original choice, but anything to keep him alive.
Aaaaaaand that never happened. Rumors! CRAZY SPECULATIONS!
SO. I reverted my choices back to the way they were and have restarted my canon game. After seeing what a non-issue the dark ritual turned out to be, I'm kind of irate that I lost Alistair for nothing. Almost took a wrecking ball to my world state to get Alistair back or maybe have Loghain live through all that anyway. Cooled off, kept my choices intact. I had my original reasons for making them and so shall they remain, even when writers change course. =p