The black flag has special significance in Islam. As Joel Richardson explains in his book, Antichrist: Islam's Awaited Messiah:
In Islam there are two flags. One is white and one is black. Written across both flags in Arabic are the words, "There is no God but Allah and Muhammad is his Messenger." The white flag is called Al-Liwaa and serves as the sign for the leader of the Muslim army and is the flag of the Islamic State. The black flag is called Ar-Raya and is used by the Muslim army. It is also called the flag of jihad, and is carried into battle. One flag is governmental and the other is a military flag.(pp. 45-46).* * *Islamic tradition pictures the Mahdi [the Islamic messiah] as joining with teh army of Muslim warriors carrying black flags. The Madhi will then lead this army to Israel and re-conquer it for Islam. The Jews will be slaughtered until very few remain and Jerusalem will become the location of the Mahdi's rule over the Earth.