There is a certain amount of QI you need to level up (Go up a belt)
Black belt and up to 10th dan (Or is it custom i forgot) you need to get 1000 QI to get up another belt. But from white up to Brown you dont have to get as many matches to level up.
http://wiki.toribash.com/index.php?title=Belts ;)
Also this: http://wiki.toribash.com/index.php?title=Qi