The theory that if one partner in a relationship likes black olives, and the other doesn't, the relationship could work out. If neither party likes, or both parties hate, black olives, according to the theory, their relationship is doomed.
Bro 1: Hey bro, how do your parents get along so well?
Bro 2: Black olive theory, bro. My dad loves black olives and my mom hates 'em.
Since olives are soaked in brine and lye to give the black color. To be called or referred to as being a black olive means that you are a liar and that your character is salty.
Hey you heardabout that one gay girl who lives in crestline? Stay away from her I heard she's a real black olive.