1. noun /ˈblækbɔːl/
a) a rejection, a vote against admitting someone
Regardless how many other people may have voted to approve a candidate for membership, a single blackball will reject the candidate.
b) a black ball used to indicate such a negative vote
2. verb /ˈblækbɔːl/
a) to vote against, especially in an exclusive organization
If youre not from a moneyed, well-connected family, you can count on getting blackballed from the fraternity.
b) to ostracize
Wikipedia foundation.