From what I know, the whole system of belt ranks started with Judo. Originally a person would simply wear a uniform with a white belt and the same belt would be worn through their entire career. The purpose of the belt was simply to hold the jacket of the uniform closed. Then, sometime in the late 1800s, a Judo instructor gave black belts to his more advanced students to help tell them apart from the less advanced students. Other arts followed up on that and soon a whole colored belt system was created. Judo started it all though.