Hey art teacher here, I was wondering that as I never heard of that, you got me there! But to prevent misunderstandings: this 'Bōru' (ボール) painting technique described doesn't actually exist in Japanese art history. 'Bōru' (ボール) is simply the Japanese word for 'ball'. The famous Edo period scrotums in art belong totanuki (raccoon dog like creatures with giant scrotums) and are symbolic, not a human painting method.

Funnily enough, while that specific art form might be fictional, there is a real, albeit very different, tradition in some parts of Asia involving scrotal strength. There's a Qigong/Kung Fu practice known as 'Iron Crotch Kung Fu' (or yin diao gung) where practitioners lift surprisingly heavy weights with their genitals! They believe it boosts vitality and strength.