A Gaussian Blur Problem

I want to do this:
http://www.metacafe.com/watch/956390/photoshop_advanced_skin_smoothing_newdarkroom_com/

but for some reason, in any photo I choose, when I start blurring..the faces of the people get highlighted and the rest of the photo turns very dark.

Anyone knows why?

thanks in advance!!
( I 'll post a pic if needed)