(Sorry this is so late, but I wanted to make sure that there was something to tell with what happened in the Walden / Blaise thread)
At some point of the train ride, Blaise would have taken Daphne somewhere private to tell her about everything that happened the other day with Walden. He was so wrapped up in himself for once that he did not notice the ring on her finger AT ALL. (It must have not been that giant to him).