1) The girl is his assistant, acting like a simple spectator.
2) The two teeth Blaine grabbed were false and are removable.
3) The girl has two more, folded up in the roof of her mouth....like on hinges.
4) Blaine pulls out the two teeth while the girl feigns shock and horror.
5) While the crowd looks on the girl in amazement, Blaine hides the fake teeth he just pulled out.
5) Just as Blaine blows on her face, she pushes the two in the roof of her mouth down into place with her tongue. You can actually see this happening in the video.