The conquistadores, did genocide in Argentina, Uruguay and Chile. They were specially successful in the first two.
So, you have left countries with mainly European population, but also, a lot of mestizos. Son of European + indigenous. Usually a European man with a indigenous woman, because the indigenous males were mainly terminated.
Those mestizos, who are not African black skinned (just a bit browner than the typical euro Argentine or Uruguayan or Chilean), usually gets the nickname "negro" "negrito", by his whiter friends.
For example, at my school in Chile (a British private school), the majority of students were european white skinned, but a few were mestizos. Those mestizos, would typically get the nickname "negro" or "negrito" pretty early in the school days (just as short people would get some nickname related to his height, same for the fat one, the ginger one, etc). If his name is for example, Carlos Perez, he would be nicknamed, "El Negro" Perez. Usually, the nickname sticks till you are an adult, specially with your childhood friends.
When used outside your friend's circle, talking about or to someone else you don't know, it will usually be as a pejorative.