Cultural Dictionary
blackball A rejection of an applicant's membership in a private organization, such as a club or fraternity. The term is derived from the traditional practice of members voting anonymously on admitting new members, using either a white marble (acceptance) or a black marble (denial). Acceptance must be unanimous; therefore, one black marble in the ballot box is enough to keep the applicant out of the organization.
Note: The term is now applied generally to efforts — especially unreasonable or vengeful actions — to keep a people or groups out of organizations they wish to join.