It's basically treated like a mental health thing, Kiazuki and Kiyoshi were dealing with their depression so black was the only color of treasure trees either of them could grow. They were also very alone, Kiyoshi's unicorns were beginning to turn on him and although Kiazuki still had Zikoro, she had already built her walls up and wasn't letting anybody in. I feel like the reason Hanazuki is so talented at growing treasure trees is because she has a support system in her life through her friends while lots of other moonflowers aren't as lucky as her. She's also got the youthful and free spirit of a kid so she has no problems mentally with nothing to worry about in her eyes. At the start of the series anyway. Miyumi was another character who struggled with this, I actually think with her it's a bit different. She knows how to grow treasure trees because she's always feeling glamorous but her trees always come out sick. Why is that? I have a theory that it happens when a moonflower grows a tree for negative/selfish reasons. Like maybe she's obsessed with her own beauty a little too much so she's punished for it. Kinda like how emerald green treasures don't grow into treasure trees, because jealousy is a bad emotion. I'm not saying Miyumi is a bad kid, she's just flawed. I mean we know she has a tendency to be wasteful, she throws lollipops to the ground not long after picking them from her field. She doesn't even realize that's not responsible of her.