There was a brief period after the Civil War, during Reconstruction, in which all Confederate soldiers, officers and officials were not allowed to vote, but freed adult male slaves could. The result was the election of some Black congressmen, senators. Like Hiram Revels of Mississippi, appointed senator, and James Lynch, elected Congressman. However, in most southern states ex-Confederates soon were able to regain the vote, terrorist organizations like the KKK became successful at reducing the Black vote, and White elites , including ex-Confederates, by 1876 were able to return to power and more and more prevent the election of Blacks.

A pretty good example of this might be Mississippi Black Congressman James Lynch, who would come to office early during Reconstruction, lose his election in 1880 to James Chalmers . He then showed that Chalmers had been able to have 5,000 votes for Lynch thrown away, and was "in favor of using every means short of violence to preserve [for] intelligent white people of Mississippi supreme control of political affairs.". Lynch would therefore regain his seat for just one more term. The White resistance and suppression would continue, however. Lynch would eventually lose, and Black representatives would become fewer and fewer. Until the Civil Rights era, the last Black serving in Congress was George Henry White of North Carolina, leaving in 1901.

https://history.house.gov/People/Listing/L/LYNCH,-John-Roy-(L000533)/#biography