The funny thing about this dress is that most people (including me) would consider it to be an evening dress for special occasions only. However, on Blair it doesn't look out of place at all. It is something she would typically wear for a day out in Manhattan with Serena van der Woodsen.
Here are two (wallet-friendly) dresses I found, one of a similar design pattern and the other of a similar shape:
NB: Blair's brooch is by Alexis Bittar.