The use of Yiddish ( or any other Jewish language's) words to despectively refer to Blacks (when not actually speaking in that language altogether) is incorrect and might be also fobidden by Hallacha. The Midrash teaches not to invlove the Jewish poeple in issues that are incumbent to them. Why forbidding N-gger and allowing Schwartze? In, fact Schwartze is just the name of the color black, it has no relation to peoples, races or ethnicities and it's not a slur. But people who don't know it, would think it's an special ashkenazi slur against blacks.