how is the black box symbolic?
The black box is first and foremost a symbol of custom and tradition--the people of the village have participated in this ritual for so long that they hardly remember why they continue to do it. Despite the fact that the box is so old, whenever replacing or repairing it is mentioned, the villagers simply continue on without actually doing anything about it. Additionally, this indicates the villagers' inability to change, as well as their deep grasp on history, since the box was presumed to have been "constructed when the first people settled down to make a village here."
Another potential symbol of the box comes from a slightly more surface-level interpretation--it is black, a color traditionally associated with death, and a wooden box, from which coffins are/were made.