I once answered a question about the disappearance of Afro-Argentines, which I think might be useful here. I would add to my previous thoughts with a couple of other points. First, research on Afro-Argentines has expanded even more from when I wrote that. We now know that colonial African populations in the Río de la Plata were probably even larger than we thought just a decade ago because slave smuggling was larger than was previously realized. They have also found other slave voyages, which have been added to the Transatlantic Slave Trade Database. Second, there was a very purposeful forgetting of blackness, not just in Argentina, but in all of Latin America during the nineteenth and twentieth centuries. National identity projects constructed nationalism around myths about national heritage by emphasizing how Europeans and indigenous people combined to create "mestizo" nations. The concepts of supposedly "white nations" in South America and "mestizaje" in much of the rest of Latin America intentionally wrote out the fact that millions of people of African descent also resided throughout the Americas. This corresponded temporally to the development of scientific racism, which gave intellectual motivation to downplay an African past. However, blackness is as much a part of Latin America's identity as indigeneity. Third, I will reiterate that Afro-Argentines never went away completely. Demographics shifted, but they remained influencial upon the Argentine economy and in the realm of Argentine culture. Many Argentines today bear the genetic markers of being descended from people from Africa. So this question always kinda strikes me as odd because my answer is to say that Afro-Argentines are still there, often hiding in plain sight. There is lot of research being done at the moment searching for more Afro-influences that have been overlooked by previous generations of scholars; they are doing important work excavating a hidden past to reveal vibrant and important Afro-descended populations and their varied experiences and contributions.

I would like to compliment the answer and subsequent elaboration by /u/Legendarytubahero about Afro-Argentines by talking about Afro-Chileans.

Unfortunately, the answer to this question will be unsatisfying but intellectually honest: We don't know. The reason for this is not because the answer has been impossible to find but rather because the research in it has been practically non-existent. The social and cultural lives of Afro-Chileans after the abolishment of slavery in 1823 and before the late 20th century has been neglected by modern scholarship (something that is evident even in the Wikipedia page for Afro-Chileans, where the timeline makes a sudden jump of more than 150 years) for reasons that are still being debated. One particular argument, for example, is the difficulty in tracing Afro-Chileans past 1823. As historian Jean-Paul Zuñiga suggests, there were no large Afro-Chilean communities. Instead, the population was spread throughout the country in individual households and haciendas in which they likely assimilated into. This would make it very difficult to follow individual Afro-Chileans throughout the late 19th into the 20th century. This is not unique to Afro-Chileans, however, as any scholar in Chilean history would tell you in their search of finding specific individuals working in a turn of the century hacienda. Yet this is only one argument and it isn't enough to answer many questions about the absence of Afro-Chileans.

Early 20th century historians acknowledged the existence of Afro-Chileans in the past but attached little importance to their presence. This coincided with a push towards the foundation of a myth of Chilean racial homogeneity in the early 20th century, with Nicolás Palacio's Raza chilena (1904) being the most influential work pushing the concept. Palacio's writing echoed the historians view (or vice-versa) that the Afro-Chilean presence was of little importance, arguing (wrongly, as many still do today) that Afro-Chilean descendants were very few in Chile and that few slaves were ever brought to Chile. Furthermore, he painted African heritage as being the opposite of being Chilean, of being "white", effectively emphasizing a racialized notion of national identity which completely disregarded the wide diversity of ethnicities that the Chilean population consisted of at the time. No matter how bizarre the text actually is, Palacio created a national myth that unfortunately still persists, in which being black signifies the 'Other' (often foreigners). With the establishment of a "white" Chile in the national imagination, this was subsequently spread and this would come to erase any notion of Afro-Chileans as belonging to Chile. The enslaved peoples of Chilean history were known to exist, but just as passing strangers in Chilean history who vanished into thin air and relegated to history where they were known to belong - effectively erasing not only Afro-Chileans as individuals from the present. but their claim to be Chileans.

Within academic history, the historiographical view of Afro-Chileans began to revise itself in the mid-1970s and 1980s with a considerable amount of Chilean scholars today focusing on diverse topics related to the Afro-Chilean experience. However, the focus given on Afro-Chileans is either limited to the colonial era of the 17th, 18th, and early 19th century, or the modern day descendants of Afro-Chileans in the late 20th or early 21st century. Tracing the Afro-Chilean experience throughout the rest of the 19th century into the 20th is therefore a very ripe area for any scholar interested in Chilean history to pursue, in particular if they're looking for a unique and important area of study. There is plenty of research to be done within the field and I would love to be able to present a proper answer to this question one day.

Rest assured, however, Afro-Chileans were never truly erased. Their culture still persists in Chile to this day, most famously through the Afro-Chilean carnaval in northern Chile. The music, dances, and literature is remembered. Furthermore, Afro-Chileans as a group were recently acknowledged as its own specific culture by the Chilean government that finally include Afro-Chileans in the national census.

Sources:

Huellas de una ausencia. Auge y evolución de la población africana en Chile: apuntes para una encuesta by Jean-Paul Zuñiga in Celia Cussen (ed.), Huellas de África en América: Perspectivas para Chile (Santiago, 2009).

‘‘One of the Most Uniform Races of the Entire World’’: Creole Eugenics and the Myth of Chilean Racial Homogeneity by Sarah Walsh in Journal of the History of Biology (2015) 48: 613–639.

Styling Blackness in Chile: Music and Dance in the African Diaspora by Juan Eduardo Wolf (Indiana University Press, 2019).

El paso de los negros por la historia de Chile by Celia L. Cussen in Cuadernos de Historia (2006) 25: 45–58.