The first authorizing order I can find is dated March 15.
http://www.ehistory.com/uscw/library/or/096/1318.cfm
By the time Richmond fell, a total of two (2) companies of black rebel soldiers had been raised, and were being drilled but had not been armed.
That would be Chambliss' Winder-Jackson Battallion and Turner's Richmond Brigade. The Richmond Dispatch on March 23rd described them as "a squad of Major Turner's colored troops, neatly uniformed, and showing a good soldierly carriage"
I can find no references to your claim that they had not been armed, and what does exist indicates the opposite is true. The March 21st Richmond Sentinal reported that Chambliss' Winder-Jackson Battallion had "served on the lines during the recent Sheridan raid." The March 23rd Richmond Enquirer documents them bearing arms while in town: "While on the Square, they went through the manual of arms in a manner which would have done credit to veteran soldiers"
Tellingly, the week before, when those black soldiers had marched through Richmond in rebel uniforms, white Richmonders taunted and threw mud at them.
If such incidents occurred at all, they appear to have been few. All of the newspapers covering the black troop's parade through town said the exact opposite of what you claim and one even reported that the women of the city were sewing banners together for them to carry. Here are some newspaper accounts of the event, not one of which says anything about taunts or mud and all of which report the reception as favorable:
"The appearance of the battalion of colored troops on the Square, yesterday afternoon, attracted thousands of our citizens to the spot, all eager to catch a glimpse of the sable soldiers. The bearing of the negroes elicited universal commendation." - Richmond Enquirer, 3/23/65
"PARADE. – Dr. Chambliss’ battalion, from the Winder and Jackson hospitals, paraded on the Capitol Square last evening, in the presence of several thousand persons. The two negro companies of the battalion were the feature of the occasion, and attracted universal attention and commendation." - Richmond Sentinal, 3/23/65
"These regulars had gone up to look at their colored brethren. Volunteering would be much encouraged by the parade of Major Turner's men, which will, we hope, soon take place." - Richmond Dispatch, 3/23/65