In her politico.com column "Black Caucus: Whites Not Allowed", Josephine Hearn reports on the unwritten rule governing membership as Rep. Stephen Cohen (D-TN) experienced it.
Even though Cohen's constituency is predominantly black, and even though the bylaws of the Congressional Black Caucus (CBC) do not prohibit non-blacks from joining, the Caucus remains exclusively black, as it has done since its founding in 1969.
It seems that "practice" and "preach" are pretty far apart on this subject. I guess it was felt by CBC members that only black people are qualified to address areas of concern to black people.
Sen. Barack Obama (D-IL) is a member; perhaps he is only half-qualified.