Blindside blocks happen on plays like kickoff returns or plays where the receiver or running-back break free on a long run. Often times supporting offensive players will make b-lines toward ensuing tacklers and launch themselves full force into the unaware tackler, causing major damage.
here's a great example: http://www.youtube.com/watch?v=tggk0tvRhYI&feature=relat...
here's another even scarier example, especially given the fact that it's youth football. My kids will never play this sport: http://www.youtube.com/watch?v=vlsTpx_-tlw&feature=relat...