But iirc she wasn't described as black, she was described as non-white, and the few details of life in D11 were so over-the-top in describing 19th century plantation life that I veered away from the childish obviousness, and assumed Rue looked more like the demographic that Mexican migrant farm workers are (indigenous mix? I'm trying to find the correct term but failing).
Also, I envisioned Peeta as vaguely Indian due to the name (think Deepak, so imagine my surprise when I saw the stills), and thus operated on the idea that a future post-uber-calamity society wouldn't have regions with current kinds of ethnic striations. And it's not like the South is majority black or that interracial marriages aren't become rapidly more common. Just because D12 is supposed to be future-Harlan County doesn't mean it should literally be Harlan County.