Blackwall is too honorable man for that. Making him BI would looks even worse that Cullen or Cole. Plus he is old as Solas. Obviously it is Iron Bull.
Eliminate women.
Let's look at our men:
Iron Bull is funny guy but looks ugly. Don't deny that please. Most women prefer man that looks like usual man.
Cullen looks OK but he isn't companion.
Misterious "NO\NO" guy. He isn't companion either.
For me it is known that Black, Solas and Varric aren't LI.
Ugly thing, not party members or women.Oh so good choice!
Oh c'mon not you too... 
It's not "known" about Blackwall, Solas or Varric, unless you have some sort of inside tip. In which case, you're breaking your NDA and you shouldn't have brought it up. If you don't have an inside tip, then it's your opinion. Nothing wrong with that, but it isn't fact.
I think Iron Bull is a handsome Qunari and if he's available, I do plan on rolling a Qunari. I plan on doing it anyway, but he'll be a nice bonus.
The devs have stated that adviser romances (which includes Cullen) have as much of the romantic word budget as the companion romances.
Who???
Beauty is subjective, so please stop calling the characters some of us are interested in, and the characters that both the writing and design team put their hearts and souls into, ugly. It's fine if they aren't to your taste. I'm not all that attracted to Cullen romantically (though he's always intrigued me as a character) and Vivienne is too flashy for my liking. Sera's personality seems too rambunctious and a little too immature, so she isn't my pick for my first LI either. Yet I think they are all good looking, memoriable and striking in their own way.