All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Black banners were mentioned in a Saheeh (Authentic) Hadeeth on the authority of Thawbaan . In this Hadeeth, the Prophet, sallallaahu ‘alayhi wa sallam, informed us that three men, all of them sons of Caliphs, will fight over the treasure of the Ka‘bah, however, none of them will take hold of it. Then, people holding black banners will come from the east and fierce killings will occur in a way that has never been witnessed before. The Prophet, sallallaahu ‘alayhi wa sallam, then mentioned the Mahdi, who will appear at that time and he ordered Muslims to pledge allegiance to him when they see him. [At-Tirmithi, Ibn Maajah and Ahmad] [Al-Haakim in Al-Mustadrak, Ath-Thahabi and Ibn Katheer: Saheeh (Authentic)] [Al-Albaani: Saheeh (Authentic)]
Ibn Katheer mentioned that this Hadeeth refers to the treasure of the Ka‘bah and that three sons of Caliphs will fight over it. Then, the end of time will come the Mahdi will emerge from the east. He added that the Mahdi will be supported by people from the east who will establish his sovereignty and they will have black banners, which is a sign of dignity, for the banner of the Prophet, sallallaahu ‘alayhi wa sallam, was black.
Ibn Katheer added that the Hadeeth does not refer to the banners of Abu Muslim Al-Khuraasaani who overcame the Umayyads, but that it refers to the black banners that will come under the leadership of the Mahdi.
Allaah Knows best.