The dark spots on the dog’s testicles are due to pigmentation and endocrine reasons. If it can be wiped off with your hands, the black spots may be dirt attached to the testicles. If it cannot be wiped off, it may be due to pigmentation.
It is normal for the testicles to turn black, but if it does not turn black, there may be some problems. Generally, when a male dog is not old enough to be bred, it is usually based on whether the testicles are black or not. If it is not black but very pink, then the dog is either underage and not suitable for breeding, or there is something wrong with the testicles. Also, if a male dog with only one testicle cannot reproduce, a male dog with only one testicle is considered disabled.