Those are sweet looking. IIRC, to get the black, I kept crossbreeding flowers of opposite colors, like in that screencap, I'd take a bunch of the ones in front, and breed with the blue green near the back. Let's say they're plants A and B. I'd breed at least two sets of A+B, and 2 sets of B+A. I'd pay attention to the offspring that were darker, and breed those, then add maybe a drab orange or a dark red. I just kept crossing on that route, alternated between reds, greens and blues till a black popped up. A lot of work but it was worth it.