Roughtly 3.7% of the Slaves were brought from Africa to North America but the rest were brought to South America and the Caribbean Islands. So why is it that today when you look up the black population numbers for the US vs the black population in South America + Caribbean Islands do you find roughly 42 Million African Americans in the US but only about 35 million(ish) in South America + Caribbean.
Shouldn't the black population of South America completely eclipse the black population of the United States if 96.3% of the slaves were sent there?
Basically intermixing: Black people in the US didn't mix at any significant number with the whites until very recent times (last 40 years), and in the few times when they did (or often still do right now), always their offsprings are labeled black. Whereas Black people, let's say, in Brazil, heavily intermixed with White people AND Natives, since they first arrived to South America.
I will focus my answer on Brazil, since about a 40% of the Trans-Atlantic Slave Trade went there. The 53.4% of Brazilians in the 2000 census identified as White, 6.1% Blacks and 38.9% Pardos (Brown). DNA studies tell us that the share varies across region between (excluding the heavily unpopulated Amazonas): 56%-74% European, 15-28% Black and 11-16% Native. Let's average Black DNA share at 23%: Brazil has 207 million people, which means if Black heritage didn't mix in Brazil like in the US did we would have 47 million blacks in Brazil alone right now.
While /u/sowser explained well why the US' black people didn't mix as much and why they reproduced more numbers (plus the fact that plenty of Caribbean English Blacks finally ended up in the US), in the Brazilian (or any Latin American nation) side, intermixture also happened due to the extremely low prevalence of White women: the Spanish, French and Portuguesse often traveled without their families for work and bureaucracy related reasons, unlike in the US were people went to North America to EMIGRATE there. Aside the few rare folk who worked for the church, or asexual, or gay or whatsoever else, the white folks traveling there must had to satisfy their sexual and emotional lives extremely often with the Native and Black women, and afterwards the Mestizo and Pardo/Mulato women as well.
Of course, Brazil and Latin American also received influx of immigrants like the US did during the 19th century and early 20th, but the local population had still the majority share and the following generations would intermix as well since there were no segretation rules put in place unlike in the US.
/u/sowser had an excellent series of comments on exactly this topic in this thread.