What does blackball mean in the union?

Answer 1

To be "blackballed" means to be refused membership or co-operation, to be abjured or otherwise rejected. In union terms this can be for being a "blackleg" (strike-breaker). The term comes from an old voting system using black or white balls to denote rejection or acceptance.

Answer 2

in a union the term "blackball" basically means to be treated unfairly or "screwed over" it is a term meaning they were cheated or under cut