Oh, for crying out loud. She would have had to recognize her from the subway train. A subway train that's full of people. And, mind you, Olivia made a point later in the show that you shouldn't talk to people on the subway., something that was very in-character had everybody working with her (aside from Abby, but we know her attitude) would have guessed. So, yeah, it is a bit of a stretch that a blind person would have recognized her under these circumstances. Blind people may have better hearing and a better sense of smell, but they're not dog-level. If they were, guide dogs wouldn't be a thing. And from a writer's standpoint, the joke would not have worked with Dan or his client saying that she recognized Olivia by her voice or by her smell, because Dan pretending that it was the horse that recognized Olivia was the punchline. And cutting the joke in favor of a more realistic correction would have messed with the mood of the bit that was completely about how weird people on the New York subway are, which was the set-up for the entire rest of the episode.