OSU,
Safety fouls, like blocks below the waist, clipping, striking, kicking, kneeing, chop blocks, illegal blind side blocks, targeting, are to be called anywhere, any time.
An illegal block in the back is not considered a safety foul. Those, and other non-safety contact fouls, such as holding, should be reserved for true advantage-disadvantage situations. If it affects an opponent’s ability to make a play on the ball carrier (which includes a player about to throw a pass), or to get to, catch, or recover the ball, then a foul is warranted.
There are more things to be considered, particularly with holding. The simple fact that a blocker has a grasp on the opponents jersey does not make a holding foul. It takes more. It takes some form of illegal material restriction that prevents the player from going where he wants go, or from doing what he wants to do.
If a defender is double teamed, the blockers are given huge leeway. As long as they don’t take him down, they can twist, turn, pull, restrict, to their hearts content. If a team is willing to put two blockers on one defender, they can do nearly whatever they want.
There are a lot more such things. Those are the exact things that are reviewed during training clinics all year long, all in an effort to get better, and get consistent.
Regarding your specific question, if it is a vicious block in the back, that becomes a safety foul - unnecessary roughness, not a block in the back.
