The main reason why a dog’s balls turn black is because the dog has entered sexual maturity. The skin on the balls will accumulate a large amount of melanin due to the stimulation of sex hormones. This usually occurs after the dog is 1 year old, so the owner Don't be too nervous, this is very normal. However, if the dog’s balls are red, swollen or scabbed, and the dog’s reaction will be very violent once the owner touches it, then it is likely that the dog’s testicles are diseased, and the owner needs to take the dog as soon as possible. Go to hospital for treatment.