which one do u think is the dream sequence?

don't know have to watch and find out

I think the one where she's happily married with Louis.

They are both real.

Blair leaving the wedding alone, ofc. !
B and Chuck will end up together - 100% sure (Just like for Dan and Serena ending up together, at the end of the show)

Neither. I think all of Blair's wedding dreams take place in the church, away from fans' prying eyes, and she actually does stay married to Louis beyond episode 5.13. :P

(The part of her running away is after Blair probably learns something horrible about Louis/her baby/the car accident অথবা whatever, at the reception...but somehow 'someone' talks her into staying with her marriage to Louis for the 'long haul' after finding her at the airport.)