Why do you think there is always an emphasis on “blindness” in the story? Provide examples

In Battle Royal, blindness is used as a symbol of how those subjected to racial oppression are often not fully aware of just how much society discriminates against them.
The boys forced to participate in a blindfolded boxing match know there's something wrong about such a shabby, undignified contest, but they still go along with it anyway. As well as being scared and intimated, they've unconsciously bought into the idea that if they do whatever the white man tells them to do, then eventually they'll be accepted as equals. So they fight each other like wild animals in the dim hope that the braying, hollering crowd of white men will give them some kind of reward.
In that sense, the boys are blind—literally, because they're blindfolded, and metaphorically, because they don't yet see just how deep and all-pervasive racial discrimination in society really is. It doesn't matter how often they dance to the white man's music to please him, they'll always be treated as second-class citizens.