There is no definition. But the purpose for stating an "open field" block in the rule language is to allow a limited level of blocks to occur, in which the player being blocked does not see the block coming. That limited level is almost exclusively the immediate area at/around the neutral zone, and between the offensive players that are the outermost linemen of the "bunched" linemen. That's a generic description of the players that are typically the tight end on one side, and the opposite tackle. But it could be two tight ends. It could be just two tackles. The idea is that, after the snap, no one should expect an offensive lineman to NOT block a defensive linemen in that area from the blind side. That is unrealistic, and totally outside the bounds of the purpose of this rule. Even though such a block within this area could be violent, the chances of such a block being violent are very small, and every player in that area must expect that he is subject to being blocked like that in this area.
It is when a player is pursuing a ball carrier, or an opponent (for blocking purposes) outside of this immediate neutral zone area that this rule is intended to apply.
BSB example: A linebacker sliding down the neutral zone to the outside while he is tracking the ball carrier and gets blasted from the blind side by a wide receiver coming back inside.
BSB example: A defensive end holding his ground, awaiting a BC that is advancing toward him, and a slot back heading back inside blasts the DE from the blind side.
BSB example: Offensive guard is chasing an opponent to attempt to block for the ball carrier and is focused on that opponent when another opponent blasts him from the blind side. (The rule applies equally to both teams.)
No Foul example: Middle linebacker steps up into the NZ immediately after the snap, and is reading the play when an offensive tackle (from a normal tackle position) blasts the MLB (who doesn't see him coming).
By and large, if you see a block that is "forceful" and from outside the opponent's field of vision, it will be a foul. If it is in that mass of bodies in the neutral zone area at or shortly after the snap, the chances of that type of forceful block happening would be very unusual to begin with, and won't be a foul, if it does happen.
Hope that helps.
Robert
