The swelling of the testicles and the back up of blood withing the scrotu caused by a back up due to lack of orgasm.
The blue tint of the scrotum is caused by the blood in the scrotum lacking oxygen. Hince the misleading term "Blue Balls". The testicles do swell up 50 percent of the size. Pain is similar to being kicked in the testicles.
When vasocongestion "Blue Balls" happened to Hue G. Rection, testicles increased 50% in size and oxygenless blood filled the scrotum. Leaving him in a dull pain similar to being kicked in the testicles.