The practice known as blackballing refers to a voting process used to include or exclude new applicants for membership in secretive gentlemen's clubs such as the Elks, Eagles or Masons. Under cover of darkness or some other cloaking method, voters would place either a small white ball or a small black ball into a communal ballot box. Depending on the established voting rules of the organization, a single black ball could disqualify a candidate from membership or a certain percentage of black balls would have to be reached.

Blackballing is generally seen as an anonymous and unambiguous voting process which leaves no doubt as to the outcome, but protects individual voters against retribution for a negative result. Some fraternal organizations, such as the Masons, use a black cube instead of a ball to eliminate any possible misinterpretation of the vote. Blackballing can also be used to vote out established members who have been accused of rule violations or other conduct considered detrimental to the integrity of the organization.

The origins of blackballing are said to trace back to the ancient Greeks, who used light or dark seashells as voting ballots. The Greek name for these shells, ostrakon, forms the root of the English word ostracize, which literally means to shun undesirable members of a society or group. Blackballing during ancient times often meant a complete stripping of all rights and privileges, plus the added indignity of being exiled from the community.

In a modern sense, blackballing may refer to an unspoken practice of not hiring an employee fired by another company under contentious or controversial circumstances. That employee may find himself or herself unable to find other local firms willing to hire someone whose reputation precedes him or her. Some incidents of corporate or social blackballing may be retaliatory or abusive in nature, while others are meant to warn potential employers or clients. It can be very difficult for a person who has been blackballed to regain his or her credibility or reputation.

Blackballing can be a vindictive practice or an abuse of collective voting power, but many organizations take steps to insure a single negative vote does not overrule the general will of the voting majority. A second ballot may be taken at a later date, or a private discussion amongst voters could result in a reversal of the original blackballing decision.