During reconstruction congress feared that pardoning former confederates would result in the suppression of black rights what happened to reinforce those fears?

Congress were fearing suppression of Black rights when the Black Codes happened.

The Black Codes were a set of laws approved by the Southern States in the Reconstruction Era in order to replace the slave codes without much change. In these laws, African Americans were to be forced to do labor on low wages. These also restricted Blacks from voting, getting an education and receiving equal treatment under the law.