It appears they wrote a story where some people with dark skin are part of an evil empire that's oppressing a fair skinned "blood witch." So, the author is being pilloried for indulging reverse racism by having some black characters oppressing a white character (although reviewers note that there are multiple ethnicities in the evil empire, it isn't just black people) and for anti-Semitism as having magic based on blood is apparently support of blood libel. That's the best I can find. It all appears to have come from
one person's tweets on the subject. Perhaps unsurprisingly, it's another trad. author going on the attack.
This book is apparently coming out in
November. Four months of Twitter hate mob review bombing and frenzied posting for her debut novel!