The Gm' says the sun never saw the P'kimah of the moon. If it would it would have chalishat Had'as. This gemarah seems to be mashmah that if the sun would see the p'kimas halevanah it would be embarrased.
However, how in metzious is it possibe for the sun to see the p'kimah, isn't there only a p'kimah in the moon because that part of the moon is not seen by the sun, i.e it dosen't see the "light of day"? Are we to learn from here that part of the moon lost its ability to reflect light?
Of course you are right that for obvious physical reasons the sun never shines on the dark side of the moon. The Gemara is giving a philosophical explanation (read: Drush with moral value) for WHY Hash-m created the moon thus.