Thank you, but why so anon?
Blair/Jack was never shown on screen, only that it happened on New Year’s. The hints began at the end of 2.14 (“he can’t know what happened on New Year’s”) and then in 2.15. Then the promos (shorter, extended) even implied that this was going to be revealed in the episode (“Blair wouldn’t touch you” “you should ask her about it”), but those scenes disappeared when it aired. It was only revisited in 2.25, when Gossip Girl announced it at Nate’s graduation party along with other bombs (like Chuck sleeping with Vanessa).