Hazrat Mahdi (as) will have black hair

According to accounts narrated by Abu Abdullah Nuaim Ibn Hammad from Abu Jaffar ibn Muhammad ibn Ali Al-Baqir, Hazrat Ali (as) was asked of the characteristics of Hazrat Mahdi (as) and he answered as follows:

The light of his [Hazrat Mahdi’s (as)] face rises to his head and the blackness of his hair. (Ibrahim Suleymanoglu, Mehdilik ve Imamiye [Mahdism and Imamate], p. 153; Iqdud Durar)
 
... His [Hazrat Mahdi’s (as)] hair hangs over his shoulders. The light of his face gives him grandeur. He has black hair. He has a black beard. (Mari’ bin Yusuf Karami Hanbali, Fawaid Fawaid al-Fikr fil Mahdi al-Montazar)

The light of his [Hazrat Mahdi’s (as)] face shines like day over the blackness of his hair, beard and head, and gives him grandeur. (Al-Uqayli, An-Najmu's-saqib fi Bayan Anna'l Mahdi min Awladi Ali b. Abu Talib Ale't-Tamam wa al-Qamal)