Abraham Lincoln issued "The Emancipation Proclamation" in 1863, during the Civil War. This proclamation declared that the slaves were free, but since the Civil War was still going on, the southern states, in rebellion and wanting to secede from the Union, did not obey the decree. The 13th Amendment to the U.S. Constitution officially abolished and continues to prohibit slavery. It was passed by the Senate on April 8th, 1864, passed by the House on January 21, 1865, and adopted on December 6, 1865.
The Flannery O'Connor short story was published in 1955, long after slavery was abolished, but a century of southern slavery was much harder to eradicate in the South. Prejudice remained long after the Civil War was over. The Jim Crow laws lasted until the Civil Rights Era of the 1960s. These were laws that, among other things, made it hard for Blacks to vote by imposing steep fees that the poor Blacks could not afford to pay. These laws had the effect of preventing them from voting in an insidious way.
The way Blacks are referred to in Flannery O'Connor's stories, including this one, is very realistic. She herself explained that while she was not prejudiced and believed slavery to be evil, nevertheless her characters were based on what she knew, growing up and living in the South. She is often criticized for using the "N" word in her stories, but again, she defends herself by pointing out that her stories do not condone slavery and prejudice against blacks. Prejudiced characters in her stories are presented in a negative and critical way, if one really understands her writing and goes beyond the surface.
Slavery was officially abolished in the United States on April, 8, 1864 with the passage of the 13th Amendment. Of course this did not prevent the mindset of those who still felt that African Americans should be subservant to them, so the battle for equal rights for them has continued throughout the 20th century. Even though Earlier in 1862 the Emancipation Proclamation had been passed by Abraham Lincoln to provide the legal framework to free the slaves, it was felt that some might view this as a temporary wartime measure and hence the 13th Amendment was passed to officially outlaw slavery in the United States of America