This play takes place many years after the end of the series.The only problem I have with her is that she appears to be rather old to be playing Hermione. I've never read the Potter books, but is Hermione ever defined as definitely white?
Whether or not Hermione was described in the books as white, there is an entire movie series that depicts her as white.
This is a play. The part will be played by many women. Having the part played by a black woman will certainly separate the play from both the books and the movie.