I'm reading this again. Zhiye started their relationship when he preened Liu Xin when they were kids. They made a point in the story to show that he is an exceptional preener and everyone wanted him to preen them when he was a child.
Zhiye giving Liu Xin blow jobs is drawn pretty similarly to when he was preening Liu Xin as a child except he doesn't look happy when doing it. But clearly he does it because he guiltily feels like it's the only thing he can do for Liu Xin.
Meanwhile, Liu Xin is in love with Zhiye and doesn't want to lose him so he made sure Zhiye's feathers stay black so he has low status and will have to stay with Liu Xin and do what he wants.
They both care about each other but Liu Xin doesn't trust Zhiye to stay with him and Zhiye so insecure and beaten down that he hasn't realized that he loves Liu Xin too.
It's super sad.
So Liu Xin continued to make Zhiye eat the special flowers to keep his wings black so he would be ostracized and have to be dependent on him. And then made him do sexual favors because Liu Xin was the only one who could "spin silk"
Dang, that's messed up.