He comes from a nazi family so he.... DOESNT want to fight black people?
You dumb
Nazis believes in hierarchy of races : the lighter you are the better human you are for them.
That's why white people with blonde hair people are seen like the pinnacle of humanity, perfect specimens of soldiers.
Thus, the Nazis don't consider darker people worth fighting ... but that they should be exterminated.
That is why Sanji never fought black people I guess