David Blaine doesn't have a box in front of him:
http://www.youtube.com/watch?v=H91syNqcIp0&mode=related&search=
He also goes higher than you could go by standing on your toes. Plus, you can see under his feet. I don't think Blaine is using this technique:
http://www.magiclearn.50megs.com/photo2.html
Do a google search on balducci levitation for more info.