In the scene where Ivy discusses her secret love interest with Blair, but says she will not reveal his name, the piano in the background clearly bears the Baldwin name. Ivy's love interest is William van der Woodsen, played by William Baldwin.
This is the sixth episode where Blair has a dream in which she is a character that was played by Audrey Hepburn. In this dream, she was Regina Lampert from the movie "Charade" (1963).