You might like to consider the way in which the narrator explicitly refers to darkness as being the dominant mode of his brother's way of living towards the beginning of the story after he reads about his brother's arrest in the paper. Consider the following description that we are given of the younger generation and the way that they are living lives "filled with rage":
All they really knew were two darknesses, the darkness of their lives, which was now closing in on them, and the darkness of the movies, which had blinded them to other darkness, and in which they now, vinditctively, dreamed, at once more together than they were at any other time, and more alone.
The narrator thus himself defines Sonny as living a life of darkness, but he himself realises how this darkness is something that he himself has experienced and continues to experience because of, for example, the death of his child. It is therefore incredibly symbolic that at the end of the story, Sonny is shown to be playing his music, which gives relief from this darkness, in an "indigo light," showing the opposition between the two states of darkness and light.