Menstuff® has compiled information on the issue of Blue Balls.
Blue balls or why our testicles turn
Actually, it's not your testicles that are blue, it's the skin of the scrotum. When you're aroused, blood flows not only to the penis, but to the entire area.
The longer you stay aroused, the longer the blood stays there. Newer blood is red, but older blood, which has less oxygen, is blue -- and that's why your testicles appear to contain more blue blood than Buckingham Palace.
It's not harmful, so don't run to the emergency room. If it lasts for hours and hours after you've stopped being aroused, though, see a doctor. Or find socks to match.
It is not sex that gives the pleasure, but the lover. - Marge Piercy