Before the Civil War, some Africans were buried in the fields of the plantations or, in some cases, in the family cemetery of the prison warden if the warden recognized the deceased as a blood relative. After the Civil War, when African Americans were allowed to establish their churches, cemeteries were established at the same time. The exact methodology of who was buried where varies by location.
For the most part, church members would buy family plots. Deeds issued by the church allowed members to waive their rights to the plot to give to a cousin or an aunt who didn't have a final resting place.
Many churches have kept these older records of burials as well as memberships and sermons. Membership records are ideal sources of information because they can lead to other relatives who lived in the area and belonged to the church.
Smaller churches in very rural areas might not have a church secretary or services every Sunday. Elders in the community usually have the names and phone numbers of the pastor, the secretary or a deacon of the church. They will be happy to share the information with you. Obtaining tax records from the IRS might also be another option if you can't contact a knowledgeable person in the community. A FOIA (Freedom of Information Act) process could take some time to have the results returned to you. Larger churches are usually fully staffed with full-time personnel, so your chances of a speedy result are greater.
If the church does not have the records you are looking for, you'll have to visit the cemetery itself and conduct your investigation. Please refer to the cemeteries page for more information.