Honestly, I know this is going to be unpopular with you lot, but I do have to say that black Hermione is bullshit. Sure, the books technically never mention her ethnicity, but they do mention it whenever even the most minor characters aren't white. It's kind of a stretch to say that the likes of Dean Thomas, Kingsley Shacklebolt, Lee Jordan and Angelina Johnson all get their ethnicity specifically stated, but it would never be mentioned if one of the main three were a minority. Hell, seeing as non-white characters never have their race explicitly stated, there could well be not a single white character in that entire franchise. Honestly, it just strikes me as retconning a white character to be a minority just for the sake of looking all tolerant and progressive than anything else.
On the other hand, it could just be a case of the best actress for the role happened to be black, which would be perfectly fine in my book. Hell, just so we're clear, I've no problem with the idea of Hermione being black, it's just the arguments that she could be black, canonically speaking, are kind of dumb.