new spoilers guys (Spoilers)
I don't think they're actually getting married. Chuck will probably sweep in last minute or Blair will realize she can't spend her life pretending she doesn't love someone else more than Louis and ditch the scene. And it will end with Serena telling Blair that everything will be okay because "they'll always have each other<3".
I'm still convinced Chair is endgame. I just can't imagine Blair marrying Louis! I'm curious how the writes are going to work it out, since it's probably the last season.