Blue balls is a familiar term for a male condition called "vasocongestion" (congestion of fluids) in the testicles and prostate accompanied by testicular pain. A prolonged and unsatisfied sexual arousal is usually the cause of blue balls. When a man gets aroused, blood flows to his penis and testicles. Without ejaculation, the pressure builds up and causes pain, mostly to the testicles. The name "blue balls" comes from the blue color of blood when it has been drained of oxygen.