It depends... It is probably just a pigment that is developing in his skin as he gets older. Kind of like a Jack Russell or Dalmation who develop more freckles and pigment in their skin while they are maturing. A Ridgeback, of course doesn't have freckles but they do have a deep color to their skin and coat often with points of darker color around apendages.
If the skin is smooth and calm and healthy looking, I would worry, if it looks thickened or elephant like then I might take him in to the vet, he may be developing allergies. I am assuming he is intact and hasn't recently gone through surgery? Are you showing him?