There's no such thing as a blue orchid or a black orchid! If someone is trying to sell you one, it's a hoax. There are red orchids, green orchids, purple orchids, white orchids, and every other color you can imagine; there are even many fragrant orchids. But blue and black? No dice.
Orchids simply don't have the genetics to produce a blue pigment. I did once see some Phalaenopsis for sale that had been dyed blue, but if they rebloomed they'd have been white orchids, and looking at the roots you could see where they'd been watered with a blue dye! Some of the more bluish purples are sometimes described as "blue," but such shameless marketing doesn't deserve to be rewarded. The closest you'll find to a blue orchid is Vanda coerulea. If you want a truly blue flower, try irises, lobelias, campanulas, or something. But this site is about orchids, so I'll leave it there.
You'll also never see a black orchid, for the same reason: orchids don't have the genes for a black pigment. There are dark-colored orchids that are almost black, such as certain Cymbidiums, but there's always a strong underlying hue. Sorry to disappoint! Orchids create dark colors by having so much of another pigment that it darkens the flower. Orchid breeders are trying hard to create closer approximations of black, because a black orchid would be really cool! Some are even pretty close. But none are really black.
Maybe they'll someday make some genetically modified blue or black orchids. That's the only way I can think of to get a true blue or black. But alas, there's no such thing at present.
Fortunately, there are other types of orchids that come in any other color you can imagine. Enjoy :)
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