عَنِ ابْنِ عَبَّاسٍ رَضِيَ اللَّهُ عَنْهُمَا أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ خَطَبَ النَّاسَ وَعَلَيْهِ عِصَابَةٌ دَسِمَةٌ
It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) addressed the people while he was wearing a black turban. The well-known meaning of "Rasimatun" is "shiny." It is possible that the turban became oily due to the use of oil.