There are no such thing as black true percs.
There's black onyx percs, black
A. polymnus, and black ocellaris.
Black onyx percs (true) are all black with three white stripes and orange on the head, fins, and tail. They usually cost $150-200 per pair.
Black ocellaris (false perc) are all black with three white stripes and maybe some orange on the head. https://www.saltwaterfish.com/site_11...4b53e5630da44c
Black A. polymnus have a totally different body form. They're black with three white stripes with yellow on the head and pectoral fins. Should be about $15 or so each.
If you ask them the price that's a pretty good way to know the species already. I'll bet they have black ocellaris, aka black false percs, but mis-IDed them as true percs. Black onyx and black polymnus are available as captive breds, but they're not as easy to find.