If there was darkness at noon, this would be a contradiction and indicate that something was wrong. This is what Rubashov feels at the end just before he's to be executed. The Communist Party had become a government that put mass society before the individual. The individual had to suffer for the greater good of society, according to Communism. They also had come to believe that the end result justifies whatever means you had to use to achieve it. So if hundreds or thousands had to suffer to achieve an ideal of the Communist Party, that was acceptable to the Communists. They had begun to use logic as a guide to human affairs and to justify their immoral behavior. To Rubashov, all of these beliefs were contradictions. It did not make sense even though he had spent 40 years serving the Communist Party. He had envisioned a revolution where both the individual and the masses would be respected instead of having to choose one over the other. So at the end, Rubashov sees no future for the Communists as they existed at the time because there were too many contradictions. Darkness becomes a symbol of what Rubashov sees as the death of the Communist Party because it can't succeed with the conflicting beliefs it has. He correctly predicts that it can never work.
Darkness at Noon represents the philosophical and political ideal of the revolutionary ideology and social morality, cannot possibly function or work. Koestler says, —“Wherever [Rubashov’s] eye looked, he saw nothing but desert and the darkness of night”—
This represents the emptyness and bleakness that Koestler say if a socialist state used increasingly totalitarian means. Under Koestler’s analysis, it appears unlikely that an authoritative revolutionary model for a totalitarian system can result in a just state. This is what the title represents.
The sun is a large, foreign body which reaches the apex of its power at noon, bathing individuals below with warmth and enlightenment.
The communist movement is like a sun-god unable to make good on its earlier promises- when its government reaches the peak of its power, it offers no warmth or enlightenment (despite the earlier hopes of its intellectuals and philosophers). Thus, individuals under its weight are forced to wander in confusion and alien remoteness, setting the tone and central journey in Darkness at Noon.
In the synoptic gospels (the Bible) the sky is dark for three hours (ending at noon) when Christ dies. This fits in with the religious imagery of the book, especially the idea of Rubashov as a messiah-figure, willingly dying for sins that aren't his own.