What does Yong Er mean by painting blood plum blossoms on a plum branch?

In fact, Yong Er's childhood favorite Bai Meihua can be seen from the first episode. The blind fortune teller told the later emperor that a member of the family who owned the plum blossom tree would become a great hero to save the country and the people. The emperor thought it was Yong Er's father and killed his father. On the night of killing Li Yuanhao, Yong Er witnessed the whole process of his father's murder in the box. Finally, he cut Li Yuanhao's knife, and his blood just splashed on Bai Mei, dyeing Bai Mei with blood plum. The blood plum blossom represents the hatred of the man who killed his father and the feeling that he will surely avenge the Li family.