First of all, we should remember that for most of the "mid-1800s" (however one defines this) African Americans in the South were mostly enslaved rather than simply suffering discrimination. However, even those African Americans who were not enslaved were discriminated against.
There are many examples of this discrimination. Free blacks in almost every state (even in the North) could not vote. In much of the South, it was illegal to educate any African American, even ones who were free. Many states had laws that allowed African Americans to be enslaved as punishment for crimes. This was not something that could be done to whites.
In these and other ways, blacks were discriminated against even if they were free.