I believe it's entirely possible that you saw a color morph that appeared to be black, when in reality as someone else mentioned it was likely a very dark green. There are BTA's called chocolates that are sometimes very dark. They always have bright white tips. LAReefs.com offers them frequently. You might ask them if they've ever come across one that's super dark. I've seen some very dark grey Ritteri with whitish tips, but not truly black and I doubt the store would have had Ritteris under HO bulbs. When I'm looking for expert anemone directive, I typically turn to ibluewater.com. If a black BTA exists, they'd know about it. They also have the chocolates and I believe they call them snow clusters or snow covered or something like that. Good luck. I hope you find what you're looking for!