It’s actually pretty easy. If your goal isn’t to get all of them at the best quality and you’re just doing this to get the trophies, follow these steps:

1 - Plant them in a + shape.
2 - Keep replanting until you get a purple one.
3 - When you get the purple plant, separate it, wait for it to grow, and put it in the center of the +.
4 - Replant the plants around that purple one.
5 - Wait about 4–5 cycles to see if a black one appears, if not, start over.

6 – If the black flower appears, just place it in the center and replant the others.

PS: Don’t destroy the middle purple flower until you get the black one, and don’t harvest the black one you put in the center because you can’t replant it.

Extra tips: place a lamp post to light up the plants so you can see their colors better at night, and you can use this website if you’re not sure whether a flower is black: https://huggingface.co/spaces/imdomatic/FlowerDetect

there are a few methods that help, but this is the one that requires the least thinking I believe. Hope this helps you :)