Mofuku were originally white--in fact, people in some regions still wear white Mofuku--but, as black has taken root as the color of formal dress after the Meiji period, and the black Western clothing has been recognized as the mourning dress, Mofuku is black in general.
本来は白いものであった（現在でも白い喪服を用いる地方もある）が、明治以降黒＝礼装の色と定められたことと、洋装の黒＝喪という感覚の影響で現代では黒が一般的である。