Maybe this has to do with french occupation of the Rhineland using black troops during the treaty of versailes.

My translation from google and if I read the letters correctly (a big maybe).
If anyone can correct or improve I'd welcome that.

The black disgrace
So a French artist represents the
rape of the breed.

Printed on reverse: 'hamburgischen landesverband gegen die schwarze schmach'.
According to google translate:
?? federation against the black ??

P1020098.JPG (119.67 KB, 180 downloads)