This is an example of irony because Old Man Warner thinks that to be civilized, the town needs to keep the lottery. It's ironic because they are killing people in a barbaric way.
The black box symbolizes tradition. The town is following an old and barbaric ritual. The box is starting to get old, but they won't get a new one, just like they won't stop the lottery. Also, the block is black to symbolize death.
The imagery of the sunny day is ironic because the ending of the story is so horrific.