"Well, that was later in the war. This reenactment is early in the war. So...you don't fit. I'd recommend coming back some other time. So...uh...nobody gets uncomfortable with someone like you around."

NTA. Tell the old racist to fuck off and come play a French African soldier. We had them far before even the start of WWI, and in very large numbers (~200,000 IIRC). And they had very interesting uniforms (although they quickly switched to the standard uniform for obvious practical reasons).

Black people didn't really serve in WW1

Yeah, that's what racist history “buffs” try to convince themselves off.