Here's a desciription of a "magical negro" from TVTropes.com. You certainly can argue that this character is NOT a magical negro, but calling those who feel it is way off base makes no sense.
"one will step forward to help a "normal" person, with their pure heart and folksy wisdom"
"They are usually black and/or poor, but may come from another oppressed minority. They step...into the life of the much more privileged (and, in particular, almost always white) central character and, in some way, enrich that central character's life."
"With such deep spiritual wisdom (and sometimes — though not always — actual supernatural powers)"
The character could have been played by an actress of any race (and surely will be when this goes regional), and the producers could have easily protected themselves by casting a white (Swedish would be extra funny) actress for the part. But I doubt they could have found an actress as good as Grant and wouldn't it have been unfair to deny the best actress the opportunity to star on Broadway because of her race?