Having worked as a sales representative for a wholesale nursery specializing in perennials, I found myself referring to the color of "black" flowers as "horticultural black". The epithet covered the deep maroon of the center of Hibiscus trionum and the deep brown flower spikes of Pennisetum alopecuroides "Moudry" (the black fountain grass) as well as several other goth flowers that don't come to mind just now. "Horticultural black" was a catch-all color that helped garden center buyers market the plants without misrepresenting the true color of the flowers. Just a short addition to the commentary. Brook Klehm