From the artigo from the link that said "Couples 2009" above. This is what they say about CB. Seems interesting and yet heartbreaking:
#6 Chuck and Blair
“Three words, eight letters. Say it and I’m yours.” Blair and Chuck are definitely the most interesting part of Gossip Girl. Their chemistry is electric and yet, they are probably the one couple on this list that could be together but (most of the time) mutually agree that they shouldn’t. Both believe their passion comes largely from messing with one another’s lives and neither can give up their self-absorbent lifestyles. It seemed Blair was willing, but Chuck can’t seem to give up his old playboy ways and potentially will never change.