At what point in "Cathedral" does the narrator's preconceptions about blind pople start to change?
I would want to argue that the narrator of this story does not actually change his ideas and preconceptions and ideas about blindness until the very end of the story, when Robert asks him to close his eyes and he realises that he is able to actually "see" even with his eyes closed. Up until this stage, the narrator is presented as insulting, greeting his wife's friend with rudeness and then turning the television on when he knows that Robert cannot see what is going on. As they watch the programme about cathedrals together, the narrator feels the need to describe what is happening for Robert, showing that he has a conventional understanding of blindness. It is only when Robert asks him to draw a cathedral with him, and then to do it with his eyes closed that the narrator experiences a kind of an epiphany and he realises that there are other forms of seeing that are not strictly linked to eyes:
So we kept on with it. His fingers rode my fingers as my hand went over the paper. It was like nothing else in my life up to now.
Even when Robert tells the narrator to open his eyes, the narrator keeps them shut, as he feels "it was something I ought to do." Even though he knows he is inside his house, closing his eyes and experiencing blindness gives him a real feeling of liberty and openness, as he says "I didn't feel like I was inside anything." It is thus this point in the novel that helps the narrator to radically reassess his own views on blindness as a limitation. His limited experience of it helps him to discover that in some ways blindness is a liberation.