Blue Vulva happens when the female is sexually excited for an extended period of time without climax. Normally going away on its own it can cause discomfort or even pain prior to dissapating. One can also release the pressure by climaxing / having an orgasm.
The blood vessels of the genitals swell upon sexual excitement; prolonged excitement without corresponding release causing the condition. The "trapped" blood may cause the genitals to take on a darker hue.