I grabbed some photos of these orchids from groups and I'm always suspicious about them too. The thing is they do exist, but it's like that whole thing with a photo using a dark filter - the plant isn't black in the literal sense, but such a dark purple, almost dark brown, that it looks black depending on the light. The Paphiopedilum rothschildianum and some Catasetum varieties come pretty close to it, but if you see it in real life under natural light, you'll notice there are definitely nuances. A lot of the illusion really comes from putting them in reduced lighting for the photo to look dramatic, which we do with our own phone cameras.