Well... almost. Except that it was kind of Uncle Jack (Desmond Harrington) who made it happen. He pointed out — wisely — that if Blair was married to Chuck, she couldn’t testify against him regarding that whole patricide thing.
So, what happened? Well, Chuck protested at first, saying that he didn’t want it to happen like this. Blair pointed out that they had both accomplished what they needed to, in one way or another, and that they should really do it. “You said you never wanted us to be boring,” she reminded him.
It seemed to break down his protestation, because Chuck softened, saying “I was a stupid child when I said those things. Life with you could never be boring.” Then, he got down on one knee and said, “Blair Cornelia Waldorf, will you marry me?”