2-3-10 Diagram

2-3-10, 9-4-3n, 9-4 Penalty A blindside block is a block against an opponent other than the runner, who does not see the blocker approaching.

2-3-10, 9-4-3n, 9-4 Penalty A blindside block is a block outside of the free blocking zone against an opponent other than the runner who does not have a reasonable opportunity to see the blocker approaching. As seen in the PlayPic, a blindside block with forceful contact initiated with open hands is legal, inside or outside the free blocking zone. 

2-3-10, 9-4 Penalty To indicate an illegal blindside block, the fists are bumped together at chest level.