The man's name is actually from "schwartzen" + "egger", not "schwartz" + "negger". A Negro is "neger" anyway, not "negger".

I don't speak German, but the best translation I can find is here:

"Schwarzen is definitely 'black' and 'egger' almost certainly derives from 'egge' meaning harrow or a tool like a plow. However, my understanding, which is confirmed after consulting with a resident German speaker here, the name actually means a 'plower of black fields' as opposed to a 'black plower of fields'"