Black Tulip + Black Cala Lily = Black Calla Lily and Black Tulip Bouquet. But the Bouquets page where you make them seems to think it's not a valid combination when it is. I tried the same thing with Apple Custard Columbine and Yellow Lachenalia, which is supposed to create a Apple Custard Columbine and Yellow Lachenalia Bouquet, but it also thinks it's not a valid combination. These are the correct flowers, yet the Bouquet making page denies it.

you need two of each flower

I realized I forgot that just a moment ago.

did you use 2 each of the black tulip+Black Calla lily?

The Apple Custard Columbine and Yellow Lachenalia Bouquet also requires 2 of each flower.

LOL never mind, I see someone is faster than me :)