In Chinese Peking Opera, there are four buses: Health, Dan, Jing and Ugliness, and each line has finer branches.
In the past, Danxing in Peking Opera societies was commonly known as Zhanxing, which was divided into Qingyi (Zheng Dan), Hua Dan, wudan, Peking Opera Blues and Lao Dan according to the age, personality and identity of the characters. Tsing Yi is a common name for "Zheng Dan" and is named after their characters who often wear blue skirts. It mainly plays solemn young and middle-aged women, such as Wang Baochuan in "Three Highs and Five Years", Li Yanfei in "Second Palace" and Luo Fu in "Sang Huiyuan". Most of the young women played by Hua Dan are innocent and lively or wild, such as Jin Yufu in Beating Lovers and Yan Xijiao in Wulongyuan. Wu Dan plays a brave woman, focusing on martial arts, especially the stunt "fighting". He often plays fairies or banshees in fairy plays, such as Zhang Sijie in Cash Tree and Jellyfish in Sizhou City, and also plays female generals and heroines, such as Tao Sanchun in Guagua Garden and Sun Erniang in Song Wu Beaten Shop. Beijing opera blues plays a young and middle-aged woman who is good at martial arts. Martial arts is not as good as Wu Dan's, so she doesn't have to "fight", but mainly focuses on singing and dancing, such as Liang Hongyu in Fighting Jinshan and Mu in Mukezhai. Lao Dan plays an elderly woman and sings with her own voice, such as her mother-in-law in The Tattoo of Her Mother-in-law and She Taijun in Yang Men Xia.
Mei Lanfang, Cheng, Xun Huisheng and Shang Xiaoyun were four famous Peking Opera masters in the 1920s, and they were known as "Four Famous Dancers".
As for why it is called this, I think it was designed like this from the beginning.