from Abū Maysarah, the freedman of al-ʿAbbās, who said: “I heard al-ʿAbbās say: I was with the Messenger of Allah ﷺ one night when he said,
“Look up; do you see anything in the sky?”
I replied, “Yes.”
He asked, “What do you see?”
I answered, “al-Thurayyā (the Pleiades).”
He then said, “Indeed, a number equal to them shall one day rule this ummah from your own loins.”
Al-Bukhārī remarked, “ʿUbayd ibn Abī Qurrah is not corroborated in his narration (1).”
Ibn ʿAdī also transmitted (2) through Suwayd ibn Saʿīd, from Ḥajjāj ibn Tamīm, from Maymūn ibn Mihrān, from Ibn ʿAbbās, who said: “I passed by the Messenger of Allah ﷺ while Gabriel was with him, and I supposed him to be Dihyah al-Kalbī. Gabriel said to the Messenger of Allah ﷺ, ‘His clothes are soiled, and after him his descendants will wear black.’” This report is munkar from this chain.
There is no doubt, however, that black became the distinctive emblem of the Banū al-ʿAbbās. They adopted it because the Messenger of Allah ﷺ entered Makkah on the Day of the Conquest with a black turban on his head, and they thereafter made it their symbol in sermons, festivals, Friday prayers, and public assemblies. Their soldiers, likewise, were required to wear something black, and when rulers invested princes with authority, they had to don a black garment called the sharbūsh.
When ʿAbd Allāh ibn ʿAlī entered Damascus, he did so dressed in black, and the women and boys marvelled at his attire. He entered through the Bāb Kaysān gate, delivered the Friday sermon, and led the people in prayer while wearing black.
Ibn ʿAsākir relates from a Khurāsānī who said: “When ʿAbd Allāh ibn ʿAlī led the people in Friday Prayer, a man praying next to me exclaimed, ‘Allāhu akbar, glory and praise be to You, O Allah! Blessed is Your name, exalted is Your majesty, and there is no god but You. Look at ʿAbd Allāh ibn ʿAlī—how ugly his face is and how hideous his black garb!’”
Black has remained their emblem to this very day, as you can still see on the preachers during Friday and festival prayers.