Probably comes from other biological terminology, like "holding your breath till you turn blue"...Basically means you are not allowing your body to do what it was meant to do...(I was going to end on perv note, but my fingers stopped me...)
Because when guys screw too much, their balls turn a neat shade of blue.
Just looked up blue balls, that's horrible hope it never happens to me, I've been aroused but get no sex
If your partner ever had them you wouldn't ask. By the way, they hurt too.
what colour is a bruise normally? oO