I’ve always been really bad at symbolism, but if I had to guess, I would say this was a message against racism (or hatred in general). A man is accused – rightly or wrongly – of murder and stands to be hanged in the morning. But the sun doesn’t come up and the town is black as night. The radio announces that only this town is black, the rest of the world goes on as normal, so we see that the hatred in the town is what keeps it in darkness.
At the end of the episode, other places around the world are descending into darkness as the town gets even darker, presumably indicating that if we don’t do something, we won’t ever see the light again. A bit strange, really, and not at all as enjoyable as many other episodes. Perhaps that’s the point.