Personally, I have never seen an African negro in Argentina. What the locals call negros de mierda are those with mixed native blood.
Negro (not the mierda) has become synonymous with "villero". Those who live in a villa Miseria are all negros, by definition.
Argentina has a generous open-border policy: all immigrants are welcome, and are automatically entitled to State benefits. They get free schooling, free hospitals, government subsidies, etc. As a result, the country has been flooded by a stream of Peruvians, Bolivians, Chileans, and Paraguayans in search of a better life. Incidentally, Villa 31 is 56% Paraguayan, and the Bajo de Flores villa has become a Peruvian fiefdom. Uruguay,by the way, accepts practically no immigrants.
Perhaps I do move in enlightened circles - must be my good luck. Hope it holds.