Republic Of China School Uniform Wiki

The May Fourth outfit, or Republic of China era school uniform, consists of a black medium skirt and a blue shirt with a mandarin collar. It can be in other colors as well, but this is the most famous combination.

In the painting "May Fourth Movement" by Liang Yulong (梁玉龙) a woman can be seen in the front lines of a parade wearing this outfit, which is likely the origin of this particular color combination being the most popular.