What is the meaning of Blackball?

A rejection; a vote against admitting someone.

A black ball used to indicate such a negative vote.

A kind of large black sweet, a black-colored gobstopper.

A substance for blacking shoes, boots, etc. or for taking impressions of engraved work.

A game, a standardized version of the English version of eight-ball.

To vote against, especially in an exclusive organization.

To ostracize.

Source: wiktionary.org