blindside block

blindside block, the action

A block delivered against an opponent from outside his field of vision, most often from the side or from behind, and most often on kick returns, interception returns, and plays where the ball has changed direction.

It happens because pursuit turns players' shoulders. A defender chasing a ball carrier is looking at the ball carrier, and a blocker arriving from anywhere else is arriving somewhere the defender is not able to protect himself.

blindside block, the rules question

This is the sense that matters, and the owner flagged it for exactly that reason. Blindside blocking is not a technique with a grey area around it: it is a specifically defined foul in high school and college rulebooks, added as its own item rather than being left to the general rules on illegal blocks, and the penalty is a personal foul.

**The precise wording, what force is permitted, and what is required of the blocker are deliberately not stated here, because they have not been checked against a current rulebook and they differ between governing bodies and by year.** Rules in this area have changed more than once recently, which is exactly why quoting half-remembered text would be worse than useless. Check your own level's current book, and check your league's local modifications on top of it.

The practical coaching version, which does not depend on the wording: teach return blockers to block what they can see coming toward them rather than to hunt somebody down from behind, and the foul mostly stops happening.