Prevent the civil wars. Afro-Argentines were used as cannon fodder there, and the rise of Sarmiento (who literally wanted to exterminate anything that wasn't European in Argentina, and nearly succeded. People like Sarmiento and those with similar ideals contributed a lot in the denigration and 'invisibilization' of Indians, Africans and Criollos in Argentine society.
However, a majority is an unlikely outcome, even with a POD before independence. Colonial Argentina had few reasons to import slaves (most of them were domestic servants) and while plantation-
like systems did exist to our shame in Argentina, the manpower was composed by Indians and Criollos. And most governments were opposed to slavery at least in principle. In practice, slave-like conditions happened (like the virtual slave conditions of quebracho tanin extraction, directed by British interests)
Maybe an earlier development of cotton, quebracho extraction and perhaps even worldwide popularization of yerba mate might reach the point that the Indian/Criollo population is so decimated that the importation of slaves are needed. But those developments happenned after independence. And I shudder to think what kind of dystopian events are required for such a large scale death and slave trade in 19th century Argentina... Awful things already happened on OTL, but I rather not think how broken would that kind of country be entering the 20th century.
Wiki has a
good article in the subject.