first of all for aesthetics, I usually leave a bit of hair over the scortum so the customer would not have noticed. I have certainly seen black coloration very normal as such. Kind of like black lips on a white dog, I would think that is quite fine.
I would add that if one testicle did not drop down, it would be very beneficial to have the dog neutered. There is often problems with the retained testical such as more likely to be cancer there and often it makes the dog much more "hot to trot" to the point of irritation.
Money will buy you a pretty good dog but it won't buy the wag of it's tail.