The term "blackballing" comes from a balloting procedure used to allow club members to vote on a new recruit secretly. Voters each place a black or white ball in a ballot box or another receptacle, black indicating a negative vote and white indicating approval. Once everyone has voted, the results are revealed, showing approval or disapproval without directly identifying those who declined the new recruit's membership.
In modern terms, blackballing refers to any exclusion, whether or not it occurs without the subject's knowledge. The term suggests a faceless refusal rather than a personal one, but in many cases, it may describe a situation in which the subject knows exactly who denied him access. Some organizations, such as the Masons, still use blackballing in the traditional sense to vote on new recruits, although the actual physical voting process may vary from lodge to lodge.
The rules around traditional blackballing varied from club to club. In some cases, a single black ball would be enough to deny a membership, while others required two or more votes to positively refuse a candidate. In those cases, slipping through with a single black ball could let a new member know of a potential enemy lurking in the club, but not the identity of the adversary.