As the new 11 man game took hold incorporating the guaranteed possession rule, new tactics were developed by coaches. How best to use the seven linemen and the four backs. Only the backs could initiate play with the ball. In rugby, blocking is against the rules and is called an obstruction. But new coaches were keen to try new ideas and plays, especially with the four backs.
One idea was a shadowing technique by linemen to prevent one player from getting into a position to tackling the ball carrier. This soon developed into a more formal strategy for blocking a player so he could not make a tackle. Eventually this led to the formal assignments of linemen as key blockers in the attack.
This is how an early block might have looked in the late 1890s.