As I understand it, it has to do with the pup's genetic ability to produce black pigment protein (eumelanin) in conjunction with the internal shape of the hair itself so that the pigment, properly produced, can be reflected down the inside of the hair shaft to the tip of the hair. If incorrect, the pigment 'may' only travel a short way. From what I have found, there is a specific 'triangular internal shape' that allows the color to reflect properly down the hair shaft. Without it, the color won't go all the way to the tip of the hair. This is all from ('bad'...?..LOL) memory, but I have a document with the explanation, diagram of the hair 'shapes' included...somewhere...
...very interesting. The correct hair shape will have the correct 'sheen' and 'feel' too...if I remember correctly.