The black color is caused by melanophores (dark color cells). So, does the presence of melanophores mean that we are always dealing with a black color in guppies? The answer will be : No! The dark color cells vary from dark brown to a bit bluish till really black. It's got to do with the density of the melanophores wether the dark coloration will be really black or less dark. But I also have to remind you that the base color (the color the guppy is born with) is gold. This could lighten up the black coloration a bit when the light hits the body.
Almost two years. The males are easier to get the coloration steady in each generation. It's the females that are a bit harder to get them the way they look right now. Black on a golden bodied female is rare. In this case, it's black on the saddle and in the finnage when it comes to the females.