Directors have much more difficulty with a choice like that than audiences do. On the other hand, an African American Blanche Dubois in Streetcar Named Desire makes us readjust our ideas about the play. Streetcar takes place in New Orleans at a time of strict racial segregation. Blanche implies that she was a Southern Belle and that her family lived on a plantation. It would make us look at the play in a different and probably a fascinating way.
Character is essential to theatre.