The combination of the colors black and red are widely used among Shia Muslims. Black symbolizes mourning and humility and red refers to anger and the spilled blood of martyrs. It’s important to note that both ‘Alī ibn Abī Ṭālib and his son al-Ḥusayn, pivotal figures within Shia Islam, were martyred. The combination of these colors is nevertheless almost exclusively used with the exclamation: “Yā Ḥusayn!” (Oh Ḥusayn!), who’s nickname among the Shia is Sayyid al-Shuhadā’, or Master of Martyrs.
The death of al-Ḥusayn at the hands of the Umayyad caliph Yazīd bin Mu’āwiya was a major event within the development of Shia Islam and still forms one of its most important tenets, namely commemorating his death, mourning the faith of his companions and the People of the House and doing penance (sometimes by self-mutilation) for the failure of al-Ḥusayn’s contemporaries to come to his aid. The colors black and red are therefore a major symbol within the Shia cult of martyrdom and penance.
Some examples featuring the colors black and red:
For more examples, please visit this thread on my Twitter.