This final scenario, that of the narrator allowing Robert to lead him in drawing an image of the cathedral signals a dawning realization upon the narrator to open up to experience and allow learning and understanding beyond his own self.
It starts again with movement–the physical boundaries of the two as they sit side by side on the carper. Borders are being traversed, the physical distance along with the mental distance between then. More on borders:
He ran his fingers over the paper. He went up and down the sides of the paper. The edges, even the edges. He fingered the corners.
"All right," he said. "All right, let’s do her."
Robert is outlining the space for their meeting. When he says, "(…)let’s do her." I find a reference to his fingering of the narrator’s wife’s face long ago.
Robert seems to understand the narrator’s feelings and seems to want to bridge the gap between them, help him to see beyond his world. The cathedral is a tool Carver uses as a metaphor perhaps for the building of that relationship, setting a metaphorical base. Robert’s encouraging, "Doing fine," and "It’s all right," evidence his knowledge of the narrator’s need to overcome his own mental barriers.
At one point, the wife wakes up and asks what they’re doing. The narrator appears to not be willing to share this with her as he doesn’t reply. Robert answers her simply, but he seems to see the breakthrough moment coming and does not want to jeopardize that.
I did it. I closed them just like he said.
"Are they closed?" he said. "Don’t fudge."
"They’re closed," I said.
"Keep them that way," he said. He said, "Don’t stop now. Draw."
The narrator is in the middle of a life-changing moment:
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.
But even as Robert tells him to "Take a look," the narrator keeps his eyes closed. He tells us, "I thought it was something I ought to do." Though we can’t be sure that the narrator is being honest with us, is telling us his real feelings and thoughts as he tells this story, I take this statement not as being something he would do for Robert’s sake, but for his own as he holds onto the moment of revelation.
Aside from his newly formed understanding and acceptance of Robert (and blindness), he may also be experiencing a new outlook on his own life. He feels a freedom that he verbalizes as:
My eyes were still closed. I was in my house. I knew that. But I didn’t feel like I was inside anything.
His reply to Robert’s questioning about the drawing do not refer to the drawing but to his own sense of outlook: "It’s really something," I said.