There were plenty of black slaves in South America and the Caribbean as well, actually.
Hence the "roughly." If you want to get technical there are a lot more slaves from other places too such as Europe, and then the massive labor force from China used in the Railways that helped "build" the United States of America. Regardless during the early time of "building" my examples were the majority sources.
Also, don't you only use the plural "Americas" when you're referring to the group of North, Central and South America? North Americas/South Americas seems nonsensical.
Half in error hence why it seems nonsensical, I was thinking broadly in North/South terms for just the northern-ish continent instead of by distinct region (North, Central, South America) or for both continents (North America and South America), apologies. I used "North" referring to mainly what the USA is and "South" referring to what Mexico is and everything more south from there. I should have worded that better then I did.