Why not cast a white singer in this role? Two reasons: (a) It's still not a level playing field for performers of colour, anywhere. That's just an easily proven statistical fact. If we don't want our stages to be a Whites-Only Zone, it makes sense to give a job to a singer of colour when that role is specifically written for a singer of colour. It will further that singer's career; inspire future singers of colour to say, "Hey, I could do this"; and expose audiences to the talent of artists of colour, which can make people just a bit more imaginative and empathetic. (b) SOUTH PACIFIC is about a white woman who can't bear the thought of mixing the races. Race is a major plot point. If Bloody Mary is black, Asian, or a First Nations mezzo like BC's Marion Newman (who was wonderful in GIIWEDIN a few years ago), we'll still get the basic story... Also, that performer is going to understand that character in a way Sally McWhitebread never could. (I am Sally McWhitebread,  BTW!)