To begin with, there were no mixed race units in the Union army. They were all strictly segregated. The only blacks one might have seen with a white unit would have been cooks, teamsters, or servants. To my knowledge, none of the USCT units of Banks' army were engaged at Mansfield, certainly not during the first phase of the battle to which you refer. There were black teamsters among the wagon train overrun during the day by the Confederates. Baylor may have mistaken the race of the dead man he saw, or it may have been one of the blacks attached to a white unit and wearing a uniform.