When a male is so close to the point of ejaculation (via oral sex, intecourse, anal-intercourse, or hand job)usually with his eyes fully rolled behind his eye lids due to the emense feeling of pleasure, in which his partner simply leaves him alone. His nuts procede to throb. This is simular to "blue balls", however in the context of "blue nuts" the sex must be MUCH rougher, rigerous, and/or wild. Thus explains the use of the word "nuts" - a much more vulger word than balls.
Thelma is riding Craig with such fury and lust. Craig's eyes are not functional due to the fact that they are rolled back behind his eye lids. Just as craig is about to let himself go, and cum every where, Thelma dis-mounts and puts her clothing back on. Craig is pissed, as his nuts are throbbing. They almost turn blue. This is "blue nuts" .