The script does not specify that she is black. The role was cast with black actresses — most of them overqualified for the role — in the original production, the first national tour, the London production, most of the early stock productions, and the film version. The casting notice in Variety for the original production stated that one of the roles was for a black woman.
For a character who doesn't sing, several of the women who played the role were exceptional singers, a bunch of whom had played Queenie in Show Boat one place or another. And, almost needless to say, several of them had played one role or another in various productions of Porgy and Bess. Which I mention just to indicate how limited the opportunities were for black actresses. Which we all know, but it bears repeating.