I think what's more interesting about this is it lines up basically perfectly with:

1. Slaves being freed, and in this newfound freedom, the freedom to speculate on their own futures.

2. The growth and success of black entrepreneurs in 20th century capitalism, resulting in actions like the 1921 race massacre in Tulsa, which seemed to be driven by white anger that blacks were becoming successful due to the "invisible hand of the market."

So it speaks to a period of rapid growth for black communities in the US, as well as access to education and capital, that allowed black writers to lean into black speculative fiction.