I am most impressed by fashion when I don't immediately know who has made all the pieces, and usually with Gossip Girl I would have to do a little digging. I think the costumer was on automatic pilot when they outfitted them for tonight's episode - twice during the show I knew exactly who had made pieces Serena was wearing.
In the park when those young girls tell Dan they are on HIS side, her bag - Loeffler Randall, and the during the blackout her dress is Vena Cava. I have to find some pictures.
You can find a newer version of the bag here: