The problem you would probably run into is that, starting with 2 breeders that are darker in color, they may already have some extensive inbreeding in their backgrounds. Especially if you get either of them from a breeder working on the same type of breeding project. If you found 2 dragons, darker in coloring that are not related and not heavily inbred (here I would be getting as much proof from the breeders as possible to be sure), you would have to choose hatchies from the clutches that were the darkest, then try to find other dark colored dragons that are not related to the parents and so on. That color trait is due to a genetic defect. Take a genetic defect, reproduce it through inbreeding and the defect becomes more pronounced. The problem is that you are also creating other genetic defects that will become evident sooner or later (deformities, organ problems, stunted growth, reproduction problems, blindness, light/heat sensitivities, shortened life spans, etc..... It could take a lot of searching and many generations to pass that trait strongly enough to produce solid black dragons consistantly. If you found a US breeder with a P. Barbata, it could be possible to get a very dark baby, then you'd have to find another non-related, dark colored dragon. Here again, I'd want proof of the bloodline as P. Barbata is almost non-existant in US collections. I'd say you are of the right mind, and choosing to not breed related dragons is definately the more ethical way to go. The pics I've seen of solid black dragons are really cool, but, I prefer not to inbreed my animals or to purchase from a breeder with heavily inbred lines.
Glad the info was helpful.