Why is the black population in Argentina so small nowadays? Because there was no African slavery there? No! There was black slavery in Argentina.
The freed blacks from rural areas moved to Buenos Aires. In the 1830s, the city had ~60K inhabitants, ~15K were black, so, ~1/4 of the Buenos Aires population was black.
But the black population of Buenos Aires became stagnant because they had higher infant mortality rates and because they had higher probability to be drafted to wars.
In the late 19th century, there was a large wave of European immigration. Buenos Aires became a very big city. The black population didn't grow, so they became a very small share in the long term. When the population of a group has very small %, it almost disappears in the long term due to the mixing.
I saw a video of a Brazilian hostorian telling this history. Those who understand Portuguese can see here
https://www.youtube.com/watch?v=XfVUfqrYHpg&t=291s 
