It was the practice of the Prophet (peace and blessings be upon him):
"His (peace and blessings be upon him) banner was black and his standard was white."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4812
Hadith Gradingالألبانی:حسن
Hadith Takhrij[هـ ك] عن ابن عباس. الصحيحة ٢١٠٠: ت، خط.