That sounded strange to me, since Brazil imports power from Uruguay and Argentina through somewhat small DC links, and if only them failing is enough to cause a blackout in Brazil, something was probably not configured correctly. So I went looking, and according to https://g1.globo.com/mundo/noticia/2019/06/16/apagao-deixa-m... "deixou sem energia toda a Argentina, Uruguai, Paraguai e sul do Brasil", so Paraguay was also affected. Paraguay shares the huge Itaipu dam with Brazil, and a large amount of power in Brazil comes from the Itaipu dam, so if Paraguay was affected, Brazil (mostly the south and southwest) will probably also be affected.

edit: the realtime graphs at http://www.ons.org.br/pt/paginas/energia-agora/carga-e-gerac... show a loss of around 1.3 GW of hydroelectric generation in the south region starting around 7:00 and ending around 10:00, matching the timing of the blackout.