Chuck and Blairs relationship is a major storyline throughout the entire run of Gossip Girl, and they have several significant moments together. However, the specific episode where they officially get together is a bit unclear, as they have several moments of romantic tension and conflict before they become a couple.

According to the search results, Chuck and Blair first hook up in Season 1, Episode 7, "Victor/Victrola". However, they do not officially become a couple until Season 5, Episode 24, "The Return of the Ring," when Blair chooses to be with Chuck and they make a pact to ensure they end up together.

Additionally, Season 6, Episode 9, "The Revengers," is a significant episode for Chuck and Blair fans, as this is when they finally end up together after Blair realizes that she has to admit her feelings as shes scared of losing Chuck.

Overall, while Chuck and Blair have several significant moments throughout the series, the episodes where they officially become a couple are Season 5, Episode 24, and Season 6, Episode 9.