A good theatre production is color blind with it's casting. Do you know how many POC have played Shakespearean characters that were white in the original production?
Get over it. It's a play, a play most of you won't see, based on fictional characters. They chose the best actress for the job. (As I'm sure white women auditioned for the part as well)
All this outrage. For what? I'm an HP stan, and yes when I read the books Hermione was white in my head. Mainly because she was white in the movies. But in this medium she's being played by a black actress. That literally has no effect on how you're personally allowed to perceive Hermione.
If this play had casted Dean Thomas as a hot British white man I'm sure the tone of the detractors would be very different.