Source: https://dep5u.upcloudobjects.com/blue-judo-gi/what-is-the-secret-behind-the-unstoppable-judo-champions-its-all-in-their-blue-gi-discover-why-1.html
The blue gi was introduced in Japan in 1965, after the establishment of the Kodokan Institute. Initially, it was used for training purposes, particularly among younger students. The colour blue was chosen because it represented calmness and serenity, qualities that are essential for a judoka's mental approach to competition.
One notable aspect of the blue gi is its use as a visual identity for martial arts schools and instructors. A well-dressed instructor can instantly convey their authority and expertise, setting the tone for an effective training session.