I answered a similar question the other day, so I'll give my answer here again:

To start I should point out that if there were popular black separatist movements (as defined by wanting US land to be made a separate country) before the 20th c, I am not aware of them. However there were definitely pre-20th c efforts to promote the physical separation of blacks and whites by either sending blacks to another country or by having blacks move to their on region/areas within the US. Both ideas, but particularly the first, were promoted by white and black advocates, with the first's most famous supporter being the American Colonization Society, which sent blacks to live in Liberia. After the failure of Reconstruction, there were also several movements of African Americans moving "out West", especially to Arkansas, Texas, and Oklahoma, where they tried to form their own communities separate from whites. Both types of movements continued into the early 20th c, and emigration to Africa has remained a very real phenomenon in the African American community to this day. But these are predecessors and cousins to the kind of movements I think you're asking about.

The first prominent 20th c proposal of the idea that African Americans should get US land that would be made into an independent black nation came from the communist movement. Communist and socialist ideas started spreading in the black community in the early 20th c, and by the 1930s (after the fall of Marcus Garvey's black nationalist movement, which promoted both emigration and black economic independence), communism had numerous black adherents in the north and south, in urban centers and rural communities. And one of the main ideas being circulated in the communist movement was that blacks should get their own state. However, American communism faced heavy repression--most famously in the 1950s era of McCarthyism--and the black coommunist movement in particular did not last as a cohesive movement. There were many accusations by black communists that white communists were either being racist or did not put enough effort into helping African Americans.

The most popular 20th c call for a US land-based black nation came from the Nation of Islam. Interestingly, the black nation idea was not part of the NOI's original teachings--when the group first appeared in the early 30s, they taught that at some point in the near future Muslims ("Asiatics", since according to their teachings all non-Europeans-descended peoples are Muslims) from around the world would invade the US and destroy all whites. However in the mid-to-late 50s the group's leader, Elijah Muhammad, started saying this cataclysm could be staved off if blacks were given their own US land to form their own country.

It is not entirely clear why Muhammad started saying that. He perhaps was inspired by the recent obtaining of independence by several African nation's. It is possible too that he was influenced by the communists. There is evidence suggesting that in the early 1940s the NOI was incorporating a few black coommunist ideas/symbols, and we know some former black communists had started attending NOI meetings and even joining by the 1950s, including the outspoken Queen Mother Audley Moore, who claimed that she and her sister were big influences on the thinking of Muhammad and Malcolm X. These communists joined during a period when the NOI was gaining mass popularity; by the early 60s it had well over 10,000 members and at least 100,000 sympathizers. Malcolm X was in the papers and on TV frequently, he spoke at ivy league schools, and the NOI had one of the most popular black newspapers in the country. Never before had a black group promoting a separate state achieved this much prominence.

But although the separate state idea officially remained part of the NOI platform through the early 70s, Muhammad never put together a cohesive plan for obtaining it. He wanted nothing to do with white politicians, whom he didn't trust, so there was no committee formed to seek a political path, and he refused to call for revolutionary action. He wasn't even particularly clear about what the exact region he wanted was--typically he implied the whole Deep South, but he was inconsistent about which states, and at one point even said Harlem should be its own black state.

In any case, the NOI started experiencing setbacks. Famously Malcolm X left the group in 1964 causing significant membership decline. Malcolm somewhat kept alive the separate black state idea, saying that if whites did not give blacks through equality through political means, he would lead a revolution. He started getting alliances with African nations and international revolutionary figures, but was assassinated in February 1965. The killers were NOI Muslims, but the FBI and CIA did all they could to foment NOI hatred of Malcolm. In fact, well prior to his departure from the Nation, the FBI had already started a campaign to reduce the NOI's influence via harassment and getting the press to publish negative stories on the NOI (although, to be sure, the white press did not need the FBI influence to make it critical of the Muslims--it already was). After Malcolm's death, people continued to stay away from the NOI for a few years due to both their disgust and fatigue with the group, and the FBIs COINTELPRO activities that put an even darker light on the group. In the early 1970s, the NOI had a revival, but this was based not on its separatist message, but more on 1) disenchantment with radical groups like the Black Panthers (who had faced even harsher COINTELPRO repression, but also had a lot more violent incidents, and then in the early 70s turned towards electoral politics to the chagrin of many early suppoerters) and 2) it's increasingly successful business and educational enterprises. By 1974, Muhammad had all but abandoned the separate state idea and was telling his followers to stop blaming whites for their situation in America--if they "do for self", he said, they would have success in America. After he died in 1975, his son Wallace turned the NOI into a decentralized Sunni community that did not promote a separate state idea.

There were a few other exponents of the separate state idea since the early 60s, but they were all influenced by the NOI. The most prominent group was the Republic of New Afrika, formed in 1968 largely by individuals who had direct contact with Malcolm and Elijah (including Malcolm's widow and the ex-Communist Queen Mother). The group developed a clear plan, created "consulates" throughout the country, and got several prominent black nationalist supporters. However its following was never large, and FBI raids led to its eventual collapse.

The RNA had multiple imitators, including Muslim ones (yes, Muslim groups were influenced by the RNA, which itself was influenced by a Muslim group), and I am aware of a few other small efforts, but none would come close to having the prominence and popularity of the NOI and Malcolm of the early to mid 60s, and therefore frequently died of their own inertia even when government repression was not involved.

Sources:. DonaldCunnigen,“TheRepublic of NewAfricainMississippi,”inBlackPowerintheBellyof theBeast,ed.JudsonL.Jeffries (Urbana: University of Illinois Press, 2006), 93–115; Raymond L. Hall, Black Separatism in the United States (Hanover: University Press of New England, 1978); Imari Abubakari Obadele, Foundations of the Black Nation (Detroit: House of Songhay, [1975]); Dan Berger, “‘The Malcolm x Doctrine’: The Republic of New Afrika and National Liberation on us Soil,” in New World Coming: The Sixties and the Shaping of Global Consciousness, eds. Karen Dubinsky, Catherine Krull, Susan Lord, Sean Mills, and Scott Rutherford (Toronto: Between the Lines, 2009), 46–55; EdwinS.Redkey,BlackExodus:BlackNationalistandBack-to-AfricaMovements,18901910 (New Haven:Yale University Press, 1969); Wilson Jeremiah Moses, Black Messiahs and Uncle Toms: Social and Literary Manipulations of a Religious Myth, rev. ed. (UniversityPark,pa:PennsylvaniaStateUniversityPress,1993); Richard Brent Turner, Islam in the African American Experience, 2nded.(Bloomington:IndianaUniversity Press, [1997] 2003); Patrick Bowen, A history of conversion to Islam in the United States, vol 2 (Leiden: brill, 2017); Theodore Draper, American Communism and Soviet Russia: The Formative Period (New York:Viking Press, 1960); Joseph R. Starobin, AmericanCommunisminCrisis,19431957 (Cambridge: Harvard University Press, 1972); Minkah Makalani, IntheCause of Freedom: Radical Black Internationalism from Harlem to London, 1917–1939 (Chapel Hill: University of North Carolina Press, 2003).