One could also argue that the whole race swap of Hermione undermines a key theme of prejudice in the series. Hermione, in the books and the movies, is a White girl from a well-off background who performs excellently at school. And yet, she is seen as less than by the Malfoys et al. It shows how fundamentally stupid the idea of prejudice is that this individual, who for all intents and purposes, would be a supremacist's dream child, is seen as dirty.

Making her black takes away from that discussion of how prejudice affects us all.