i was just wanted to know what dan you have to be to write a black belt

The Koreans copied the belt rank system that the Japanese were using several decades ago. The Japanese do not consider anyone under the rank of 3rd dan qualified to give out rank. Even then the person must alos have been issued a written Sensei title. So basically only someone of at least 3rd degree black belt and also having the Sensei title can promote. Then that person can only promote up to two ranks below his own. Korean styles tend to bend the rules often. But no legitimate martial arts instructor is going to accept a rnak given by a 2nd degree black belt. It just is not done.

Source(s):
Martial arts training and research over 42 years (Since 1967).
Teaching martial arts over 36 years (Since 1973).