It seems like there are regular requests for black roses or for roses that are so dark that they appear black.

Possibilities:

Try the book, Black Plants: 75 Striking Choices for the Garden, and if you can't find something you like in there don't despair because florists are adept at coloring carnations - black is probably possible and carnations should work as a substitute. Lastly, like the little black dress, silk works too.