GenZedong

(I'm going to use the word "African American" a lot because these organisations use it, even though I find "Black Americans" a more correct term.)

Obviously no such thing as true emancipation of some without full emancipation of all and all that, but you probably want more meaningful answers than slogans.

Black Nationalism movements had to struggle with this question before, with middling results. Nation of Islam itself used to promote the formation of a sovereign African-American nation-state somewhere in the South, and Malcolm X's views developed from separation to nationalism as he split from them. The so-called "New Black Panther Party" (not to be confused with the now-called "Black Lion Party") is one of the few organisations that proposes such a thing.

Racial separation has many issues as a central proposition from a theoretical standpoint. First off, going by Stalin's definition of a "nation", racial groups in the USA as it stands today lack the key aspect of common territory, and so are in shaky ground as a "nation". While creating such a territory as an after-fact might sound like it corrects the issue, it actually makes the settler contradiction quite clear.

Suppose the NBPP manages to somehow acquire and defend some territory in Louisiana. Not all African Americans live in that region, it's not common territory to anybody but the (black) people who live there. So now you have to develop a system in which black people from the whole of the (possibly quite hostile) USA can migrate there, but even then, these "foreign" African Americans will not have the same historical connection to that specific region. No matter how much legislation is passed and proclamations are made, it'll take a reasonably long time for these "foreign" African Americans to integrate, and in reality they'll be immigrants in everything but name. Besides that, many African Americans for many reasons might not be able to -- or might not want to -- immigrate, and for those people this new state presents no solution to the systemic racism in the (possibly quite hostile) USA. In effect, it proposes that the only salvation for African Americans is to flee their homes, their lands, their neighbourhoods, their history, to some isolated Garden of Eden. It invalidates African Americans outside this new state, similarly to what Israel does to non-Israeli jews, and gives an excuse for persecution abroad.

Now, this state doesn't exist inside the USA political system. It's gonna need its own army, its own diplomatic organisation, its own intelligence agencies and so on. If the relationship with the USA is adversarial, they'll be forced into concessions or risk being invaded. It does not solve racial conflict, it merely simplifies it to the stage of state diplomacy and dislocates the people from the equation.

Obviously this deals mainly with black people in the US, who don't have specific "legal" historical claims to its territory like other groups like Chicanos, Indigenous people and such, but it shouldn't matter. By not having control of their lands, they've all been dispersed all throughout the USA and abroad. Achieving nominal independence might be a temporary benefit, but so long as the original US is still allowed to exist and to exert its own racism within its borders, it'll do all it can to control this new country. And they'll probably have an easier time using it as a pressure valve.

This is why Black Nationalism rejects the idealistic notion of a Black nation-state as the solution to racism, but the key word here is "solution". Separatism can be a tool, which may be used in the struggle for broader political goals. The Land Back movement commonly ends up in just petit bourgeois land transfer, but it has also provided some experiences of popular self-government and wealth redistribution to more radical organisations (Nick Estes talks a lot about his). If the NBPP was primarily socialist like the old BPP and came to the conclusion that separation was the correct tactic for the emancipation of black workers, that'd be interesting. However, they in principle focus on cultural/racial separation first, and a lot of their non-Marxist positions follow from that. It's why they do militias first, free breakfast programs second, and barely any union work.

Since you mention the UK, separatism there is also mostly more successful as a tactic than a strategy. There's massive rejection to Westminster's fixation with austerity in general -- and New Labour in specific -- in Scotland, and it's growing in Wales and NI. They also have an advantage over the US due to the common territory issue. The SNP and Plaid Cymru notably lost votes to Labour during the Corbyn years (though he really fumbled in Scotland with Brexit), and regained their popularity now with Sir Kid Starver. The Alba Party is a good case study for failing by trying to use Nationalism and Independence, but having no clear coherent proposals that require that independence in the first place. NI is a whole can of worms.

That's all to say, secession in the USA could be an useful tactic for "unpermitted" policy but is a bad strategy, and Lenin himself probably has some text about that. AFAIK it's unlikely as I don't know of any regions with strong enough political movements outside the permissible spectrum, so some fringe progressive groups get shock headlines about calling for independence and nothing else, and thrash meaningless online polls sometimes get huge margins, but it's not going to actually happen unless they seek something that's impossible while staying in the US and have the local support for it. Despite all the fear-mongering, the USA is not nearly geographically polarised enough. There's no "Yellow State" that is dominated by a local party that can't possibly the win National Elections like in Scotland. A "PSL Independent Illinois" would be an interesting development, I guess.