In the beginning, the tone is one of skepticism and a hint of jealousy. The narrator doesn't understand why she would want to spend so much time with a blind person. The narrator simply is not able to empathize with the blind man or understand what it is like to be blind. Later in the story, the narrator tries to describe a cathedral to the blind man:
The narrator's description of the cathedral is inadequate partly because of the blind man's impaired vision, but largely because of the narrator's own blindness to spiritual and human values. (Enotes)
However, when the blind man takes the narrator's hand as the narrator draws the cathedral, he finally has his epiphany in the story (an epiphany is a moment of enlightenment). The narrator has finally been able to think of ANOTHER, and not himself:
[This experience] evokes a deep, rich joy that transcends the superficial relationship they expected to establish, transforming the narrator from a vain, selfish person into one who, in thinking of another, finds himself. (Enotes)
The tone, at the end, then changes to one of realization and enlightenment.