I still don't see how duty somehow precludes Blackwall from romance. Heck, Anders repeatedly tells Hawke that his cause for the mages is more important than his love with Hawke, and he ended up blowing up a building over it, too.
For all the reasons Wynne lectures you about romancing companions. Love is selfish - if you have a choice between saving the world or the one you love, what do you do?
Had my Warden not romanced anyone, I probably could have done the ultimate sacrifice. But since my Warden love Alistair, and couldn't bear either of them dying, she talked him into doing a ritual with unknown consequences. I made that decision knowing that I could have just completely screwed over the world, but "that's a problem for another time, at least we're both alive."
A strong sense of duty would mean not wanting to put yourself into a situation that could comprise your duty. Which love would do. It could make someone who was close to their Calling, and accepting of their fate, suddenly no longer want to die.