Praise be to Allah.
The hadith mentioned in the question was narrated by the great Sahaabi Abu Hurayrah (may Allah be pleased with him) from the Prophet (blessings and peace of Allah be upon him) who said:
“There will emerge from Khurasan black banners which nothing will repel until they are set up in Aelia.”
Narrated by Nu‘aym ibn Hammaad in al-Fitan (1/213); Ahmad in al-Musnad (14/383); at-Tirmidhi in as-Sunan (2269); at-Tabaraani in al-Mu‘jam al-Awsat (4/31); al-Bayhaqi in Dalaa’il an-Nubuwwah (6/516); and Ibn ‘Asaakir in Tareekh Dimashq (32/281)
All of them narrated it via Rishdeen ibn Sa‘d, from Yoonus, from Ibn Shihaab az-Zuhri, from Qubaysah ibn Dhu’ayb, from Abu Hurayrah and attributed to the Prophet (blessings and peace of Allah be upon him).
At-Tabaraani (may Allah have mercy on him) said:
No one narrated this hadith from az-Zuhri except Yoonus, and only Rishdeen narrated it from him. End quote.
Al-Bayhaqi (may Allah have mercy on him) said:
It was narrated only by Rishdeen ibn Sa‘d from Yoonus ibn Yazeed. End quote.
This Rishdeen is the reason why this hadith is regarded as da‘eef (weak) and munkar (odd). He was classed as da‘eef by Ahmad ibn Hanbal, and Yahya ibn Ma‘een said concerning him: His hadith is not to be written down. Abu Zar‘ah said: His hadith is da‘eef. Abu Haatim said: His hadith is munkar (odd), he is negligent, he narrated odd reports from trustworthy narrators, and his hadith is da‘eef. An-Nasaa’i said: His hadith is to be rejected. Abu Dawood said: His hadith is da‘eef. Ibn Hibbaan said: His reports are mostly odd.
See: Tahdheeb at-Tahdheeb (3/278)
To sum up: the hadith is da‘eef jiddan (very weak) and is not saheeh, because Rishdeen ibn Sa‘d was the only one who narrated it; hence the scholars did not accept it.
At-Tirmidhi said: It is ghareeb. End quote. This means that he regarded the hadith as da‘eef.
A similarly-worded report was narrated from Ka‘b al-Ahbaar. End quote.
In fact, when al-Haafiz Ibn Katheer narrated a number of hadiths that mentioned the black banners, he said: All of this leads one to doubt the soundness of these hadiths, as none of their isnaads is free of problems.
End quote from al-Bidaayah wa’n-Nihaayah (6/248).
The hadith was classed as da‘eef by al-Haafiz Ibn Hajar in al-Qawl al-Musaddad (42); Shaykh al-Albaani in as-Silsilah ad-Da‘eefah (no. 4825); Shaykh Ahmad Shaakir in his commentary on Musnad Ahmad (16/316). The commentators on Musnad Ahmad (Mu’sasat ar-Risaalah edn.) said:
Its isnaad is da‘eef jiddan (very weak). End quote.
In fact they also said:
There is no saheeh report about this matter; there are only da‘eef (weak) and mawqoof (i.e., the isnaad does not go back to the Prophet (blessings and peace of Allah be upon him)) reports. End quote.
Similarly, Shaykh Dr Haatim ash-Shareef said:
There is no saheeh, marfoo‘ hadith about the black banners, and there is no hadith that has an isnaad going back to the Sahaabah (may Allah be pleased with them).
End quote from a study on hadith published for the first time on the Islam today website, then published in Multaqa Ahl al-Hadith
Anyone wanting to know more detail on the evaluation of the hadiths that speak of this topic may refer to the study mentioned above.
Whatever the case, al-Haafiz Ibn Katheer (may Allah be pleased with him) said that, even if we assume that the hadith is sound,
These black banners are not those with which Abu Muslim al-Khurasaani came and overthrew the Umayyad state in 132 AH; rather it refers to other black banners with which the Mahdi will come. End quote from al-Bidaayah wa’n-Nihaayah (19/62)
It should also be noted that Khurasan is a large territory that includes the north-east of Iran; it extends from Jarjaan and Tabaristan on one side, to Transoxiana on the other side. From a political point of view it includes Transoxiana and Sijistan (now Afghanistan). The region that is now known as Khurasan encompasses less than half of historical Khurasan; the rest of that region now belongs to Afghanistan.
End quote from Ta‘reef bi’l-A‘laam al-Waaridah fi’l-Bidaayah wa’n-Nihaayah li Ibn Katheer (1/471)
And Allah knows best.