The black community in To Kill a Mockingbird is portrayed as a separate community within Maycomb. Taking place not long after the Civil War, in the South, memories of the Confederacy's ignoble defeat are fresh in the minds of those who are still alive to tell the story, as well as those who keep the anger over the defeat very much alive.
Blacks are considered to be beneath the whites. Even Bob Ewell, who is an outcast within his white society, holds himself with a sense of superiority over Tom Robinson, specifically, and all other blacks, in general.
Ewell's ability to charge an innocent Tom Robinson of raping his daughter Mayella is possible because many of the prejudiced residents of the community believe it is possible: the accusation represents the true nature of their discrimination and fear of blacks. Truth is not found in the voice who speaks it, but in the color of the skin of the accuser and the accused.
One of the big mistakes Tom Robinson makes in the story is to feel sorry for Mayella's lot in life. How could a lowly black man feel sorry for his superior? This is, in fact, quite possibly the detail that seals his fate.
Yes, suh. I felt right sorryfor her, she seemed to try more'n the rest of 'em—"
"You felt sorry for her, you felt sorry for her?" Mr. Gilmer seemed ready to rise to the ceiling.
The witness realized his mistake and shifted uncomfortably.
We cannot say for certain that Tom really had a chance at a not-guilty verdict, but as we learn later, there was a good deal of argument among the jury members, so the decision was not an easy one. Based upon what some members of good conscience saw happen in the courtroom, they felt compelled to give Tom the benefit of the doubt. However, they were overruled.
The one person who is black and has a different place within the community is Calpurnia. The bottom line is that the children view their housekeeper through the lens of how Atticus sees her: she is like a mother to them, and no different because of the color of her skin.
The story seems to accurately depict the black community of Maycomb, and of the South during the Great Depression. From Ewell's vicious lies, to the lynch mob that shows up at the jail to forcibly take Tom while Atticus guards him, shows the extent to which many members of the community view the blacks. (There are exceptions, like Atticus and Miss Maudie.) Atticus takes some abuse (like Ewell spitting on him) for defending Tom, while for the children, it is much harder, coming at the hands of some adults and a good many children.
However, the bottom line in the story is the message Atticus gives his children: to know a man you must walk in his skin, and Atticus does this, and encourages his children to do the same.
He sees Tom as a man first: the color of his skin is incidental. He is realisitic enough to know how the community at large may see Tom, but Atticus' moral compass not only demands that he be fair-minded, but that he look to a person's humanity before considering anything else, and it is how he lives his life.
I like the detailed answer by the previous poster, but I still have to disagree with the idea that the black community is depicted realistically -- in the widest sense of the term "realistic" -- in Harper Lee's novel To Kill a Mockingbird.
The black community is indeed depicted as existing wholly apart from the white community, and blacks enter the white sphere almost entirely as workers in the service industries (e.g. Calpunia as the Finchs' housekeeper and Calpunia's son as the local garbage collector). Those specific details strike me as very realistic.
Beyond that, however, the black community in Lee's novel To Kill a Mockingbird is presented mostly as a type or trope that furthers the novel's theme of "good versus evil." The opening of the novel establishes that the entire story is told in retrospect; an adult speaker is looking back on her childhood with more than a hint of nostalgia. This novel does not pretend to be a historical presentation of the realities of black life in the 1930s South.
The most significant passages ni the novel that help support my point, I believe, are the descriptions of the "negro cabins" further up the road, beyond the city dump and the city itself. These descriptions clearly present the black community as cleaner, more orderly, and overall more civilized than the neighboring Ewells, a white family that lives right next to the city dump.
If you move past a surface reading of the novel (e.g. past the openly stated lesson of walking in another person's skin), you may very well come to see how the novel may be reproducing stereotypes even as it claims (on the surface) to oppose them. In this deeper reading, for example, the blacks are presented as a group of "good people" in clear opposition to the Ewells, who are presented entirely as a group of "bad people."
To me, then, the representation of black people in the novel might be more accurately described as "moralistic" (seeing things in clear-cut terms of what is good, bad, right, wrong, etc.) than as "realistic" (seing things as they truly are, in all their complexity).