(spoilers)
Is Cassandra really a Magical Negro? In some ways, she upends the stereotype. Her employers may be Caucasian, but she in no way behaves subserviently to them, and she makes that extremely clear at several points during the show. She also doesn't really "save" Vanya and Sonia, or show them the light. They don't take her powers seriously. Vanya and Sonia are not wary of Hootiepie at all.
Shalita Grant plays her with some kind of weird accent (I don't think that is the way Grant actually speaks), and the character performs voodoo, but if those two things weren't true, I'm not sure that the character would be seen as reinforcing the Magical Negro stereotype.
"What was the name of that cheese that I like?"
"you can't run away forever...but there's nothing wrong with getting a good head start"
"well I hope and I pray, that maybe someday, you'll walk in the room with my heart"