The OP writes about the 54th Massachusetts. However, there were many more black regiments in the Union Army. Almost 200,000 blacks served as Union soldiers. That is a sizable portion if the Union Army.
Basically, almost all black regiments, including the 54th Massachusetts, wore the standard Union uniform. A couple of the regiments raised by General Butler in New Orleans MAY, I repeat MAY, have been dressed as Zouaves. IIRC, there was a full corps of black troops as part of the Army of the Potomac under Grant.
It would probably be a good idea to issue a set a black soldiers for those who want a more representative Union Army.