The hadîth about the army with black banners coming out of Khorasan has two chains of transmission, but both are weak and cannot be authenticated. If a Muslim believes in this hadîth, he believes in something false. Anyone who cares about his religion and belief should avoid heading towards falsehood.
The hadîth is related on the authority of Thawbân that the Prophet (peace be upon him) said: “If you see the black banners coming from the direction of Khurasân, then go to them, even if you have to crawl, because among them will be Allah’s Caliph the Mahdî.” [Related by al-Hâkim (8572) and Ahmad (22387)]
All the chains of transmission for this hadîth are weak and inauthentic, though some people have been overly lenient about it and declared it authentic by virtue of the many chains of transmission that it has.
Some people have used this hadîth to support their claim that the Mahdî is from the family of al-Abbâs and that the Mahdî is from of the Abbasid dynasty. There were Abbasid Caliphs who went by the name al-Mahdî.
The banners of the Abbasid State were black. It is not hard to see how this weak hadîth might have been fabricated or at least tampered with to support the Abbasid cause.