Goodness, so, your going to pick apart every detail of this metaphor?why not, when they uses weakass metaphor like this one have to pick them apart.
The entire point (which it seems that you are failing to understand) is that if we all live in darkness, and there is no God, then we should never have realized the darkness that we live in.well , we dont live in darkness and to my concern there aint no god so who fail to understand what? that this dude fail to realize the obvious fact that he just rambling about a load of nonsense crap.
"if we lived in totall dark and didnt have eyes......" if man could use the brain man have
then we should never have known the concept of right and wrongman had a concept of right and wrong long before they learnt of this biblical god, I love how so called religious people think that that the notion of right and wrong is one of their inventions