The ball used in a Masonic ballot by those who do not wish the candidate to be admitted. Hence, when an applicant is rejected, he is said to be “blackballed.”
The use of black balls may be traced as far back as the ancient Romans. Thus, Ovid says in the Metamorphoses (xv, 41), that in trials it was the custom of the ancients to condemn the prisoner by black pebbles or to acquit him by white ones: Mos erat antiquus, niveis atrisque lapillis, His dammare reos, illis absolvere culpae.
the source: Mackey’s Encyclopedia of Freemasonry