The central irony of "Cathedral" is that the narrator is the one who is truly blind, in a spiritual sense. The narrator is contrasted with Robert, a man who cannot physically see. The narrator pities Robert, believing his quality of life and relationships must be far lower than his own because of his disability. However, the narrator only "sees" people in the most superficial sense. He has a hard time relating to other people or even comprehending his own inner life.
For example, the narrator is not especially close to his own wife. She writes poems to express her feelings about important moments in her life, but the narrator is unable to properly appreciate them, because he does not understand them—and therefore does not understand his wife. He is self-absorbed in the extreme, unable to see beyond his own experience, even expecting to hear his wife sing his praises when Robert comes by to visit.
The moment where Robert and the narrator draw a cathedral together is the moment the narrator's inner eye is finally opened. Realizing he cannot describe a cathedral to Robert, the act of drawing one with Robert allows the narrator to not only connect with someone else, but to engage in shared communication. It is an intimate moment in which both men see the same thing. The narrator's narcissistic barrier is broken; he can see now as Robert sees.