The darkness in the story is both literal and figurative.
The story is set in New York City. The narrator lives in an area of the projects, which is oppressive and run down.
The narrator's and Sonny's uncle was killed by white people who ran him over and there is a great hatred for them in his family. The narrator's and Sonny's mother lives in Harlem and races many hardships. Sonny is in prison for a drug conviction and an addiction to heroin. This factual information contributes to the dark, depressing nature of the story. The narrator laments Sonny's plight, being in prison for his drug addiction and wonders if his brother will ever straighten himself out. He and Sonny are disconnected emotionally and the narrator struggles to understand why Sonny did what he did.
Despite the darkness of the story, there is a hopeful epiphany at the end of the story because the narrator is able to finally understand one of the driving forces behind Sonny and his potential recovery from drug addiction...Sonny's music. The narrator realizes that Sonny's music is a driving force, a positive for him in a world of negative, and that the music is a peaceful escape for Sonny.