Having grown up in the "vivid killing streets" of Harlem, Sonny, a sensitive musical man, senses the fear about which the older people do not speak; he suffers from the despair of his environment, the confrontation and and "the darkness which roared outside":
The darkness outside is what the old folks have been talking about....It's what they endure...Some escaped the trap, most didn't. Those who got out, always left something of themselves behind, as some animals amputate a leg and leave it in the trap.
In order to deal with what Baldwin calls "the menace [that] was ...reality," Sonny shoots heroin into his veins as it gives him a feeling of being "in control," "some vision of his own," as he reveals to his brother when they peer through the window at a woman singing during a revival on the street. Sonny remains in a "loose and dreamlike" state most of the time while he uses heroin.
Playing music, also, "makes something real" for Sonny. His jazz helps Sonny release "that storm inside" him, and this is why he tells his brother that sometimes the musician will do anything to play as he recognizes all "that hatred and misery and love" that exists on the streets. For, he is able to release much of his suffering when he plays music, especially, the blues.
When the brother/narrator accompanies Sonny to the club and hears Sonny play the blues, he realizes the power of Sonny's blues to deal "with the void," and to impose order on things. About Sonny, he notes,
What is evoked in him, then, is of another order, more terrible because it has no words, and triumphant, too, for that same reason.....[the tale of how he suffers]...is never new, [but] it's the only light we've got in all this darkness.
For the first time, the narrator realizes the power of music as a release from suffering. For, after his daughter Grace has died, the brother understands Sonny: "My trouble made his real." In this realization, the narrator has an epiphany, perceiving his brother as, perhaps, a darker side of himself, and a side with which he can join in his efforts to keep out the darkness and fear of their environment.