Let me briefly recount the story of the Black Crown. When the Emperor Yung Lo met the 5th Karmapa Dezhin Shegpa numerous miracles and signs appeared in the sky for the 18 days that Karmapa gave public teachings. Famous Chinese painters and calligraphers of the period copied these signs daily and wrote descriptions of them in the four main languages of the Chinese Empire Mongolian, Mandarin, Tibetan and Turkic. They form the basis of our understanding of how the Emperor came to give the Black Crown to the Karmapa.

After several days of teachings, the Emperor said to Karmapa, Whenever you perform a ceremony of blessing, you always appear to me in a special way. Your body is in the form of Vajradhara and you are wearing a kind of black turban or crown on your head. 

Karmapa responded that it could be that when the body of a great bodhisattva is teaching in human, or nirmanakaya form, this body can also be simultaneously manifested in sambhogakaya, or ethereal form. To further answer the emperor's question, Karmapa explained that many eons in the past, in a previous life as a cave-meditator, Karmapa attained the eighth bodhisattva bhumi. Then, a hundred thousand wisdom dakinis cut their black hair and offered it to Karmapa as an offering. They manifested their hair as a crown that they placed on his head and enthroned Karmapa as a Buddha for their sambhogakaya land. The 5th Karmapa said that the 1st Karmapa, Dusum Khenpa was an emanation of that sambhogakaya Buddha for our world.

The emperor marveled at this story, and in his devotion offered to have a replica of this crown made for Karmapa: If I make a similar crown and offer it to you, can you give the blessing of the sambhogakaya Buddha to sentient beings? 

Karmapa responded, Yes, the bodhisattva s blessing depends on his having attained the wish paramita--that whatever he wishes for sentient beings will come true--so this is possible. 

The emperor was very pleased. He left Karmapa, and ordered the most skilled craftsmen to make a crown studded with precious stones and crowned by a rare ruby the size of a human fingertip. The 5th Karmapa accepted this gift and began the tradition of the Black Crown Ceremony. Since then, when any Karmapa performs this ceremony, he meditates in the form of a sambhogakaya Buddha and thus gives a unique empowerment of samadhi and wisdom to those in attendance. Karmapas from the 5th through the 16th wore this crown and gave this blessing through the ceremony.