1. Slang - To be extremely excited about something soon to happen, and become denied right before the event.
2. Actual - To be aroused to the point of extreme erection, and be denied right before actual intercourse. Testicles become bruised and pain occurs; hence the name and color description: blue balls.
1. I've been craving some MGD beer, but when I got to the bar they gave the last one to some bitch. Damn... what a blue ball.