Blue balls is a slang term for the achy feeling and bluish color some people get in their genital area if they’ve been sexually aroused for a long time. Some people notice this feeling especially strong in the testicles. Because the blood that has built up in the genital area is not returning to pick up more oxygen, it can make the skin in the area a bit bluish in colour. This cannot hurt you. As soon as the blood begins to move away from the genital area, which can happen if the person has an orgasm or is not sexually aroused anymore, the discomfort usually goes away quickly.
Some people use the idea of blue balls as an excuse to pressure other people to have sex with them. This type of pressure is coercion and is not part of a healthy consent conversation.