The sailor uniform is an icon of Japanese school uniforms. They come in a variety of colors, including navy, blue, gray, and green. White and black are not super common, which makes them stand out in illustrations. Black uniforms give off a more rigid appearance, whereas white is associated with purity.
Nezuko (from Demon Slayer: Kimetsu no Yaiba) certainly looks cute! What do you think of the other black and white uniforms below?