Among male vervet monkeys, "blue balls" are a sign of status. The bluer the scrotum, the more dominant the male.
Arno Meintjes

Is "blue balls" a real condition? What is it, and is it dangerous?

—Beth Jones '06

The blood that causes erections also fills the scrotum—hence the term "blue balls" (blood within veins often appears blue). Although it feels real, it's not a medical condition, and not something a woman needs to "treat" with sex. Women experience the same sex-organ blood congestion, yet you never hear of women complaining of "blue clitoris." If a man does not have an orgasm, it takes a little time for that blood to leave the area, causing a dull ache. It may be a little annoying to have to come down on one's own, slowly. But it's not that big a deal, and most guys have learned how to treat it themselves.

—Thanks to Michael Castleman, M.J. '79, author of 13 books on health and sexuality