Terezi has to fight to keep her serious expression because the scent of him gesturing frantically in what she assumes to be disbelief is really hilarious. She goes a step further to mimic his gestures and wave her hand in front of her own face. Obviously, she doesn't see it.

"Yes, that is what that means. I am blind. I have been for several of your human years. So peeking through or around my blindfold is not a thing that could have happened."

She taps her foot impatiently, still frowning. "Are there any other insensitive accusations that you would like to make?"