The Onion

It literally was not a slur. You are trying to burnish your progressive bonafides way too hard. Word meanings change over time.

As you mentioned, there was actual slavery happening at the time. Being called a "negro" was the last thing a slave would worry about. They wouldn't even identify as that, because they would consider themselves Ashanti or Igbo or some other West African ethnic group. It'd be like calling you "North American" (I assume).