Those Black Occs on EBay are definitely not w/c and certainly not rare,,,, I have an assload of Black misbar juvenile B/W occs ,,, the misbars are caused by the food/environment at the larval stage. Every now and again I get it wrong and end up with a batch that are nearly all misbars.
A tank-bred Black Occ will likely not be used to foods like mysis and chopped shrimp. If they're not accepting a good quality flake then try cyclop-eze or live baby brine. They'll take the other foods eventually.
Don't worry about the color, they will darken. Mine start to lose their orange coloration within a week or two of being removed from the grow-out tank.