It's because Brazil's topagraphy lent itself to large plantations that required the use of African slaves. In fact more slaves from Africa went to Brazil than the U.S.
Argentina, on the other hand has a topagraphy much like Western Europe and slavery was not as needed. Therefore Brazil's black population is very high while Argentina's is miniscule.
Argentines are mainly european stock, mostly spaniards and italians, there weren't many inidigenous people there before them, that's why they look "white" most of the time.