The wardrobe for Blair Waldorf was a character in itself. The headbands — which became, in the show’s initial cultural impact, the single most recognizable visual signature of the character — were not simply costume choices; they were narrative choices, because Blair’s headbands were a specific shorthand for a specific kind of control. A girl who wears headbands is a girl who has decided to manage something as specific as the way her hair sits. She is imposing order on something that could escape her. This is a very precise visual representation of what Blair does in every other dimension of her life.
Meester’s skill in the role was partly the skill of wearing a costume without being subsumed by it. The headbands, the Chanel, the specific shoes, the apartments — all of it could have resulted in a performance that was all external surface, a character who existed entirely in the signaling. What the performance found, and sustained over six seasons, was the interiority that the exterior was designed to manage.
Her working relationship with Blake Lively, who played Serena van der Woodsen, was the central professional relationship of the show’s run. The Serena-Blair dynamic — the push and pull between the effortlessly charismatic girl who seems not to need the social order and the strategically intelligent girl who has built herself inside it — was the show’s primary dramatic engine for much of its early run. Playing that dynamic required both actresses to calibrate their performances against each other in ways that are specific to long-running television: over six seasons, the relationship between two characters is subject to an enormous number of variations and evolutions, and maintaining the essential tension while allowing those variations requires a specific kind of professional attention.