There are so many images of darkness and lightness in "Sonny's Blues." I think there are such images on every page in the story, and I will provide a few. As to the effect of the characters' dark skin on the white world, there are not many explicit references to this in the story because, for the most part, the setting gives us African-American characters who are interacting in an African-American setting, Harlem.
In the very first paragraph of the story, while the narrator is going home on the subway, he refers to the "swinging lights" of the car and the faces of everyone "trapped in the darkness that roared outside" (20.)
In the third paragraph, the narrator refers to Sonny's face as "bright, and open, there was a lot of copper in it...(20.)
Moving along to a later section of the story, the scene the narrator describes in which he last saw his mother alive, he says, "the night is creeping up outside...," and speaks of "darkness growing against the windowpanes... (28). He refers to "the darkness coming and the darkness in the faces...(28.) Then the description shifts. He says,
"In a moment someone will get up and turn on the light....And when light fills the room, the child is filled with darkness. He knows that every time this happens he's moved just a little closer to that darkness outside (28-29).
In this passage, Baldwin is alluding to the darkness of the African-American world of that time and place, when the darkness of being African-American was certainly a consequence of the effect of the color of dark skin on the white world.
Another example of the effect of the color of the African-American on the white world, is the scene in which the narrator's mother describes how the narrator's uncle was run down and killed by a group of drunken white men, who surely would never have done this to a white man.
If you go through the story slowly, you will see references to lightness and darkness that may or may not be obvious to you because the words "light" or "dark" are not being used. For example, Baldwin uses the word "indigo" (44), which is very deep, dark shade of blue. Also notice what a nice touch this is, in keeping with the title of the story, which is about two kinds of "blues," music and sadness.
Most significantly, the images of darkness and light contain the symbolic meaning of the environment in which Sonny and the narrator have grown up. For instance, in one flashback, the narrator recalls how the parents sat in the evening as the darness grows, as "every face looks darkening" and the "child moves a little closer to the darkness outside." In another instance of flashback, the narrator recalls how the young boys live in two darknesses:
The darkness of their lives, which was now closing in on them, and the darness of the movies, which had blinded them to that other darkness...
That other darkness is what Sonny lives in and feels--the darkness of drugs, and the "storm" that lies within him which he can only get out by playing the blues, and playing to those who truly listen. The narrator finally understands Sonny as he sits in the dark corner of the nightclub:
For, while the tale of how we suffer, and how we are delighted, and how we may triumph is never new, it always must be heard. There isn't any other tale to tell, it's the only light we've got in all this darkness.
In the denouement the narrator perceives the drink of Scotch and milk on top of Sonny's piano as glowing with light and shaking like the chalice of suffering that Sonny has experienced. Finally, then, the narrator understands his darker side, who is Sonny.