The persistence of the color line in literature means that works by African American writers often appear in 'black' contexts--works categorized as "urban literature," shelved in the "African American section" at bookstores, or anthologized together in collections featuring black writings. For the most part, in the broader realm of literature, most African American writers are outliers in a marginal sense.
There are, on the other hand, a few select authors who are outliers in relation to the majority of black writers. These writers' works appear in black spaces, but they also appear in mainstream venues as well and receive attention from large numbers of non-black readers. These select black writers are outliers in the exceptional sense.
The term "outliers" became especially popular a few years ago when best-selling author Malcolm Gladwell published a book of the same name. Gladwell's Outliers: The Story of Success focuses primarily on the many factors, including birth month, accumulative advantages, and the 10,000 hour practice rule that empower select individuals to become extraordinarily accomplished.
It might be worth figuring out how a few, select black writers achieve crossover appeal and become highly successful while many black writers remain separate from mainstream spaces. That separation is not always negative nor involuntary. There are and have always been successful, admired African American writers who are not widely known outside of black circles.
Still, there's some resentment concerning the apparently small number of black writers who do attract national attention. There's frustration about the suspected reasons that so many black writers are "ghettocized" and only a chosen few are accepted. The issue of black writers as outliers raises all kinds of concerns.