The Flower Petal Blade Art was indeed a forgotten technique of the Imperial Mao. Long before, there were special warriors from the Imperial Mao that made use of dancing as their movements during a battle. There was no rhythm, but they moved like flowers on a windy day while slaughtering enemies. However, as this technique required a high level of grace and flexibility which were discouraged among men who made up a big percent of armies, only women were given the chance to study this blade art.
As the times grew less and less chaotic for imperial families, most women were barred from holding the sword or any kind of blade for combat. Soon, the number of potential wielders of this blade art vanished. It could only be stored in the library of the Imperial Mao until Mao Jixue took interest in it...
It was not because she wanted to pick up the sword for real and fight... but to show it to a struggling friend, Xu Rouguang!