ninjutsu did not have a belt system until recently. The belt system (kyu/dan ranking) was created by Jigoro Kano, founder of Judo. Pretty much almost every martial art system has adopted this system. In Japanese martial arts rank was historically given out with certificates. There was no real standard but they generally started with Shoden, Chuden, then eventually ended with Menkyo Kaiden which meant a person has "graduated" from that school.