Well, in my experience there are many different kinds of blacks- lace, melano, orchid, superblack, copper black. It all depends on how you breed them.
Orchid is, IMHO, not really a black "black," but a bi-color. It's a black betta with heavy, HEAVY blue iridescence often in a butterfly patten on the fins. It's usually the result of a marble cross. (MrV, I think this is what you're describing.) Seen here:
Lace is basically an orchid with clear edging on the dorsal, anal & caudal. They usually have a lot of blue iridescence as well. Seen here:
Melano is the attempt at a PURE black betta with zero irridescence. Breeders have tried to inbreed the mutation that spreads the melanin across all the body and fins- sort of an "extended" black. The inbreeding was so frequent that eventually melano females became infertile. The eggs are frail and rupture. There is usually zero iridescence on the fins of a true melano.
Copper black is just that- a cross between a melano and a copper. There is generally a lot of copper iridescence on the fish, giving it this appearance-
Superblack, which is one of my current lines, is the attempt at getting the betta as dark-black as possible with minimal iridescence, while keeping the females fertile. This requires an outcross to an unrelated steel blue female every few generations. You can still sometimes see minimal iridescence on the branching of a superblack, but it is greatly reduced from that of an orchid. Seen here: