The whole story is allegorical; so it relies heavily on symbols and metaphors. The black box is a metaphor for tradition. No one really knows why it's black or its true origin--there are rumors that it was made from pieces from the precedingbox, but no one seems to no for sure. Tradition is the same; most people do not know why they do something from year to year but continue to practice whatever the tradition might have been because that's the way is always been. For most traditions, there are a variety of stories toldabout their origin, but nobody seems to be sure which story is the most factual. And most significantly, when someone suggests changing or updating tradition, his or her suggestion is usually shot down (just like the one to update the black box).
Another metaphor is the color black. It represents death and "sin." The box is black; Mr. Summers who proctors thelottery works for the coal company, and the mark on the slip nobody wants is black. Everything that is negatively associated with the lottery is black. Those who do not draw the marked slip, seem to be redeemed of their "sins," as their clean slips swirl around in the wind.
One can also argue that Tessie is a metaphor for a scapegoat. She has done nothing wrong, and yet the townspeople willingly stone herbecause they always stone the person who pulls the marked slip. Their clear slips are allowed to float away in the breeze (similar to their "sins"), while Tessie must be "punished" for the entire town.
As a whole, Jackson relies upon metaphors to demonstrate the power of tradition, the force of a mob, and the idea of a scapegoat.