The discussion about this is by no means settled, so I don't think that we can have a question like this without shattering the 20-Year Rule. I would suggest /r/AskSocialScience as a good place to bring it to, or else /r/Ask_Politics.

Unfortunately I don't have the time to type out the post I wanted to (final papers) but I'll try to as soon as I have time. I'll give a shorter answer first though.

Slaves were not allowed to learn how to read in most states. The majority worked on plantations as farm hands. After emancipation, their only property was often the clothes on their backs. They were illiterate farm hands that did not have land or the ability to buy land to farm. during Presidential Reconstruction, Southern States enacted "Black Codes" essentially disenfranchising black Americans and forcing them into highly restrictive labor contracts. Things got better during Congressional Reconstruction, with some change happening. However, the Congressional Reconstruction state governments were overthrown by conservative whites. One of the ways conservative whites achieved victory was through voter fraud and intimidation. Voting was usually done at the courthouse and you had to tell someone your vote. You didnt go to a polling station, go into a private area, and write down your vote. You told someone and so people knew who you had voted for. Whites with guns would be loittering around inside and outside the polling stations to intimidate blacks.

One conservative whites took power, they cut spending to programs that had helped African-Americans, such as public schools. Within about a decade Jim Crow began, and African-Americans were essentially disenfranchised again. Whites used literacy requirements, requirements of knowing the Constitution (remember many blacks couldn't read, didn't have the time or ability to go to school to learn to read or learn the constitution), land requirements (they didn't have anything after emancipation, let alone their own property), poll taxes (if they can't read, can't go to school, aren't able to buy land how are they going to pay poll taxes).

Sources: A Short History of Reconstrution, Eric Foner State of Rebellion, Richard Zuczek Pitchfork Ben Tillman, Francis Butler Simkins