Just to rephrase the question a bit, the question is essentially whether anyone in the United States had attempted to argue for reserved legislative seats, presumably in connection with the black community.

The closest thing to reserved seats in the United States would probably be the nonvoting delegates to Congress that the Cherokee and Choctaw nations have by treat right, but that only literally just right now are anywhere near being attempted to be exercised, so that will fall outside the 20 year rule. This is a bit similar to the reserved Maori seats in the New Zealand parliament. It's also worth pointing out that the idea of a separate electorate choosing representatives is roughly applicable to tribal governments in the United States, in that, say, enrolled members of the Cherokee Nation choose a tribal government under terms of tribal constitutions, which has been a process in place since the Indian Reorganization Act of 1934 (with various ups and downs since). But these are treated as domestic sovereign nations, and don't sit in Congress beyond the potential exceptions listed above.

As for the black community, especially in the Reconstruction Era, this was not really a goal that was advocated. The obstacles towards greater political, civil and economic rights among the former slave population were not specifically caused by a lack of representation in Congress - if anything, Congress under the Radical Republicans in the Reconstruction Era was trying to enforce those rights in conflict with recalcitrant state and local authorities, and a hostile US Presidency under Andrew Johnson.

It also is perhaps a bit of a misconception to think of black Americans as a subjugated minority. While blacks were a minority of the national population in the Reconstruction Era, they were overwhelmingly located in the American South, to the tune of some 90% of the black population. Furthermore, in parts of the Deep South they were an absolute majority of the state population (this was the case with South Carolina, Mississippi, and Louisiana), or roughly half of the population (this was the case in Georgia, Alabama and Florida), or almost 40% of the population (as in Maryland, Virginia and North Carolina). Within these states, at the county and local level, many regions were overwhelmingly black, especially in the so-called "Black Belt" that had been cultivated for cotton growing using slave labor and then black sharecropper labor following 1865. In the entire 11 states of the former Confederacy, something like 40% of the population were emancipated slaves.

This is worth mentioning because it helps to explain why Reconstruction focused so heavily on equal protection under the law (under the 14th Amendment, adopted in 1868) and preventing voting based on race (under the 15th Amendment, adopted in 1870). This is also why, once Reconstruction governments fell at the state level to white supremacist "Redeemers", and new state constitutions limited the black franchise and civil and political rights under Jim Crow, much of the political restrictions were framed in terms of literacy requirements, poll taxes, and the like, rather than out-and-out voting restrictions explicitly based on race.

The African American Civil Rights movement from the end of Reconstruction to the 1960s and beyond has been mostly concerned with focusing on equal political and civil rights for the black citizens, rather than gaining separate representation per se.

Interestingly, Isabel Wilkerson has made the argument that it was precisely demographic shifts in Southern states in the mid-20th century that made white Southerners more amenable to equal political rights for their black neighbors. With the Great Migration of American blacks to Northern cities, especially during and after the Second World War, the racial demographics in Southern states shifted, so that no Southern state had a black majority any longer. 37% of Mississippi's population was black in 2010, and 32% of Louisiana's - the only entity with a black majority in that census was Washington, DC, which conveniently only has a nonvoting delegate in Congress and is not a state. Wilkerson has argued that with this demographic shift, it became possible for white Southerners to both give in to local and national demands for black political and civil rights, while maintaining control of statewide governments, and of most House of Representative seats. So for example Mississippi has four House seats, one of which has a black Democratic representative (elected by an overwhelming majority), while the other three seats are held by white Republicans, also elected by an overwhelming majority.

A final point as to why such a proposal was never seriously taken up in the United States is that the US Congress is not really constructed in the same way as a parliament, like in, say India, despite both being federal states. Each state is alloted a certain number of House seats based on population (this gets revisited every ten years after the national census), and then the state draws its own districts. Each state also has an equal number of Senators, and until 1913 these were elected by state legislatures. Edit: it's also worth noting that each chamber of Congress has the power to recognize and seat - or not - any members coming from a state, and ironically this happened at the beginning of Reconstruction, when congressional delegations made up of elected ex-Confederates were refused seating in Congress in 1865-66.

Adding voting representatives to the House would at the very least require a constitutional amendment (meaning it would have to be approved by 2/3rds of both houses of Congress, and then be approved by 3/4ths of the states). The representation in the Senate is effectively unamendable - it would require unanimous approval to change. Given these exceptionally high bars, as well as the federal nature of the system, the focus on guaranteeing black political and civil rights has long had a strong focus on ensuring equal enforcement of those rights at a state and local level, in addition to a federal one.