Blacks are about 20% of the US population, while Netflix has some demographic information I can't really imagine they are tracking race of the viewer (I could just imagine the shit storm that could erupt if they did), they do however classify films based on various categories.

Netflix didn't invent black cinema people did and way before Netflix was even an idea in some one's head here are for example Black Cinema film festivals:

American Black Film Festival Freaknik Hollywood Black Film Festival International Black Women's Film Festival The Kidflix Film Fest of Bed-Stuy North Carolina Black Film Festival Pan African Film Festival Roxbury Film Festival San Diego Black Film Festival Women of Color Film Festival

The US also has "black only" film awards including the African-American Film Critics Association Awards, Black Reel Awards, Black Movie Awards and so on.

So after watching multiple movies that most likely their only common denominator was "Black Female Directors" Netflix suggested you a category that contain additional movies that fit your recent viewing habits.

On a side note, I watched the new Crouching Tiger, Hidden Dragon and the Marco Polo special on Netflix recently and all of a sudden what would you know? Chinese / Asian martial arts films appeared and took over my suggestion feed.

However after binge watching Cosmos and The Inexplicable Universe I wasn't greeted with the suggestion of the category "Documentaries by Black Astro-Physicists" but rather with "Space and Nature Documentations".