Well if there was nothing stopping you romancing Cullen, wouldn't you say he was your preferred LI? Some might still prefer to romance character X even if they can't as the Inquisitor they are going to play and still consider them their 'preferred LI'.
Mmmmm, don't think so *strokes chin* It says "preferred LI for you" ... so I think, for my playthrough, my experience, who is my preferred LI? But... I see what you mean about the wording. "Preferred" LI. The one I WANT the most, whether I get him/her or not. So, yeah.
The "lighting up" of another characters name, after you select your option and hit the vote button, is just the name that currently holds the Top Spot (highest percentage).
When I originally voted (for Solas), Cullen lit up after hitting the submit button. I checked the numbers, and Cullen (at the time) had the highest percentage. That is all the highlighted name means. 
No blood magic, then?! Darn 