|Source: Ngram Viewer|
For one, Langston Hughes is apparently cited much more than even other canonical black poets such as Countee Cullen, Gwendolyn Brooks, and Margaret Walker. Still, Hughes and those poets tend to follow a common pattern: peaks during the 1970s and then later again in the 1990s. I've run several other tests and come to realize that the pattern is somewhat widespread.
The visibility of the Civil Rights Movement and Black Power and the development of Black Studies programs and increased enrollment of black people in college and graduate programs during the late 1960s and on into the 1970s were all central to the popularity of "black subjects."
|source: Ngram Viewer|
Ngram Viewer is imprecise in some ways because it only tells word and phrase usage and not necessarily how the terms were used. Still, it is useful for painting a broad picture. Moving forward, I'll run more tests using the tool as I move to gathering more information about black poetry over long stretches of time.
• Shifts of labels concerning "Negro poetry" and "Black" poetry
• LeRoi Jones vs. Amiri Baraka vs. Black poetry