There is no evidence that supports the claim that black imama can only be worn by Sayyids (the descendants of the Prophet of Allah SAW). The Sahabah (may Allah be pleased with them) used to wear imama of various colours including black, white, and many others. Therefore, it is permissible for non-Sayyids to wear black imama as well.
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دلائل:
مصنف ابن ابی شیبۃ: (رقم الحدیث: 245)
حدثنا سلیمان بن حرب قال حدثنا جریر بن حازم عن یعلی بن حکیم عن سلیمان بن ابی عبداﷲ قال: ادرکت المھاجرین الاولین یعتمون بعمائم کرابیس سود و بیض وحمر وخضر وصفر…
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.