There's a lot of places in Africa and Latin America that the US could annex with the intention of resettling freed men but that's never going to happen. The freedmen don't want to leave and the southerners still need them. America is still reliant on its cotton production and cotton production is still reliant on the exploitation of black labor. There is nobody to fill that gap with resettlement. The Irish aren't going to do it by the hundreds of thousands.
It is expensive to move hundreds of thousands of people to overseas colonies, especially if they're unwilling to go. You're going to pay for hundreds of ships to do hundreds of trips across the Atlantic or Caribbean, having to feed the "cargo" along the way, and setting up infrastructure for housing, food supply, and water by the time they land? For what? And what happens to the natives? We saw what happened to Liberia. Multiply that by a factor of 100.
Even if the US goes the route of colonization with the mindset of the upcoming generation of European imperialists, whether they take the Congo or Namibia or even places in the Indian ocean they're not going to be settled with more than 5 figures and it's highly unlikely they become full fledged colonies like otl's European counterparts in the same territories regardless. So no massive American infrastructure projects in the Congo like a Grand Inga Dam, probably.
It might be easier not to free them and just sell them to Brazil. Wouldn't that be a nice chapter in our history books? In a parallel universe where Hitler gets to read about it in the early morning of January 20th, 1942, he might even skip the meeting that day and just stay in bed and put on his favorite jazz albums.
The best bet is carving out a state or states within the US's borders but where are you going to do that? California and Oregon don't want them. Nevada, Arizona, and New Mexico are deserts. The Mormons don't want them so there goes Utah and Idaho. The Dakotas are cold, Montana and Colorado are a bunch of mountains, cowboys, and natives, and Oklahoma has other plans. That just leaves Washington as a territory and there you'd have a second trail of tears and resettling a bunch of former slaves in a place named after a guy that owned slaves. And you're going to piss off the Oregonians and they'll come up with a reverse fugitive slave act if anybody tries crossing South from the Colombia or the 46th. Not to mention dropping off six million people into a land that's got only seven million people today in mostly very densely populated cities.
The alternative is where the slaves outnumbered the white populations in the south, but you're going to need to do some serious tampering in post civil war politics to even consider that and possibly light the Constitution on fire.
Scale ca. 1:3,000,000. LC Civil War Maps (2nd ed.), 13.2 "Entered according to Act of Congress, A.D. 1861 by Henry S. Graham." "Sold for the benefit of the sick and wounded soldiers of the U.S. Army." "Census Office, Department of the Interior, Washington, Sept. 9th, 1861. After a careful...
www.loc.gov
You could make a few small autonomous regions that aren't exactly states and classify them as reservations. Though that seems offly messy, but given the subject you're asking about I guess it kinda goes with the territory.
If you have to give a whole state to freedmen just make Charles Sumner president and he'll trail of tears the whites out of South Carolina and give the freedmen a home there, though the slave population in 1860 was higher than South Carolina's entire population today. Anybody remember his Crime Against Kansas speech?
"Were the whole history of South Carolina blotted out of existence, from its very beginning down to the day of the last election of the senator to his present seat on this floor, civilization might lose — I do not say how little ; but surely less than it has already gained by the example of Kansas, in its valiant struggle against oppression, and in the development of a new science of emigration. "
Yeah, he took quite the beating for that one.
1851: "The Crime Against Kansas" -- May 19, 1856
www.senate.gov
I root for the President Sumners option. Hell, he might also annex Canada if the Brits take his Alabama Claims more seriously. "Oh you meant that?!"