What is Raymond Carver up to in the story Cathedral?

Greetings from Carver Country (Raymond Carver graduated from the high school I started my teaching career at in Yakima, Washington, where he grew up.  His dad worked in the Boise Cascade lumber mill that only recently shut down here).

As with many of Carver's stories, there is tension and darkness.  The narrator, who drinks a lot (many of Carver's characters do) and uses drugs in the story, is a little upset that Robert, a blind friend of his wife's, is coming to visit.

The story is tight and subtle.  Carver liked to leave the endings to his readers to figure out for themselves.  I think carver is trying to get the reader to look through the eyes of the narrator as he undergoes a fundamental change in how he sees life.  He gets to this point by talking to Robert and witnessing the way he sees life, and deals with his blindness.  But Carver doesn't tell us this directly.  At the end of the story, Robert and the narrator are drawing a cathedral on a piece of paper with their eyes closed.  Simulated blindness, and what the narrator can see in his mind.