My textbook states that they would draw attention to prejudice in the US and claimed black people would be treated as equals in Germany, though none ever surrendered. Would the Germans have kept their word?
I don't know about this but if you are curious I actually know of the Japanese doing this during WW2 in trying to create a rift between the black and white American servicemen. It was one of the only times Japan actually used mostly the truth in their propaganda (they had a tendency to rather go with blatant lies) and was seen by the higher ups in the U.S. Military as a major threat because of the reality of the racial tension. Since of course at this time there were also race riots happening back in the U.S. And the Japanese propaganda even referenced this.
I'm sorry I can't help with the ww1 propaganda though.