The play -- which features beloved Harry Potter characters Harry, Hermione and Ron as adults working at the Ministry of Magic -- is based on an original story by the 50-year-old author.
Rowling tweeted that she "loves black Hermione" on Monday in response to a fan who asked about the casting choice, and added that "white skin was never specified" for the original character. Actress Emma Watson played Hermione in the hugely successful film adaptation of the series.
"Canon: brown eyes, frizzy hair and very clever. White skin was never specified. Rowling loves black Hermione," Rowling's tweet reads.
Rowling also retweeted actor Matthew Lewis -- who played Neville Longbottom in the film -- who wrote, "And Neville Longbottom was blonde. I really don't care. Good luck to her."
Meanwhile, Dumezweni -- who's certainly qualified for the role, having won the Olivier award for best performance in a supporting role for her work in 2005's A Raisin in the Sun -- played coy at the big announcement about her casting on Sunday.
"Um... So. Just seen my feed BLOW UP... What's THAT about?!" she joked.
Check out Dumezweni alongside actor Jamie Parker -- who will play adult Harry Potter -- and actor Paul Thornley, who nabbed the role of Ron Weasley.
Harry Potter and the Cursed Child -- which takes place 19 years in the future, picking up right after the final book's epilogue -- opens in July 2016.