I think that the black box helps to add universal meaning to the story because it is the element that holds or contains the violence that is associated with "the tradition" in the village. The tradition is only possible with the black box and the name pulled out of it. The pieces of paper that remain in the black box helps to add to the arbitrary nature of the horror that results. The black box contains the names of all the people in the village. The name that gets pulled becomes the victim to unspeakable violence. On some level, the black box is symbolic of the almost arbitrary nature of the tradition and of horrific violence, in general. When we consider the violence that exists in our world and the almost random nature of it, these incidents could be representative of a "black box" of "lotteries" where someone's name is "pulled" at any given moment.