The location for the concert was not chosen for its audience capacity. Anderson had tried to book Constitution Hall, but the Daughters of the American Revolution, which owned the hall, refused to let her perform there because she was black.
First lady Eleanor Roosevelt interceded and arranged for the alternate venue." (more from NPR Weekend Edition Sunday, April 12, 2009)
Marian Anderson was one of the leading contraltos of the 20th century. If you haven't heard her, please give a listen: