The black box is a very cool symbol because it reveals so much. First off, it is worn and tattered. This completely symbolizes the practice of the lottery. They perform the lottery only because it has always been done, not because it is an actual scientifically proven fact. They believe it like they believe proven facts. No one has ever not done the lottery as long as Old Man Warner has been alive so they have never questioned or hypothesized anything different.
The box being black foreshadows that the lottery may indeed be something evil. This black color may suggest that something terrible will happen because of this lottery.
Both of these features ultimately come true or bear moral lessons that the reader later learns when the last sentences are read.