It was used in place of black for a longer period, and wasn’t necessarily considered a slur in and of itself. But of course if you say it with a sneer, even “black” can be used as an insult.
For example a lot of books (even written by people of color) used “negro” and “coloured” etc. interchangeably up to the mid-late 20th century. But in modern context very few people use it in a manner that isn’t derogatory.