What did Congressional Republicans' think of the South's Black Codes?

The Black Codes were essentially an attempt by Southern states to reintroduce the antebellum hierarchy. Southern states wished to retain the substance of slavery at a time when its outward form had been abolished. The passing of the 13th Amendment, which ended slavery, caused widespread alarm in the South. Many feared that the newly freed slaves would seek revenge on their former masters and undermine the established system of white supremacy and control.
The enactment of the Black Codes was the direct outcome of this fear. Southern legislatures passed a series of laws that placed restrictions on African Americans in terms of where they could live, where they could work, and what kind of work they could do. They also sought, in some cases, to deny African Americans the right to vote and hold public office.
Such an egregious violation of the spirit of the 13th Amendment caused widespread anger and disgust in the North, especially among Republicans in Congress. The authority of the federal government was being seriously undermined by Southern state legislatures, who seemed to not realize they had lost the Civil War.
The response of the Republican-controlled Congress was swift. Over President Johnson's veto, they passed the Civil Rights Act of 1866. They also passed the 14th Amendment to ensure equal protection of all American citizens, irrespective of race. Additional rights were also given to freedmen to allow them to participate in the government of the Southern states. The Reconstruction Acts were passed by Congress to tighten federal control over the defeated Confederate states.
This was the high point of Republican control in Congress. However, as evidenced by Johnson's numerous vetoes and general foot-dragging on the issue of civil rights, we see that not all Republicans were quite so enthusiastic about Reconstruction. For one thing, they were uneasy about the spread of government power, seeing it as no more than a temporary postwar measure. As far as the less radical Republicans were concerned, the outward forms of repression had been abolished, and there was no need for any additional action. They were quite content to leave enforcement of civil rights laws to the Southern states, despite their outward hostility to the very idea of equal rights in the first place.
In due course, the Republican Party grew tired of Reconstruction, as did most Americans. Although still maintaining the justice of the abolitionist cause, Republicans effectively abandoned African Americans, leaving them at the mercy of Southern white supremacists who proceeded to undo civil rights laws and subject them to the evils of Jim Crow.
Why did this occur? There are two possible reasons. One possible reason is that Republicans were indeed committed to formal equality but not substantive equality. In other words, they, like virtually all white Americans at the time, did not believe that African Americans were equal to whites in terms of culture, learning, or intellect. In that sense, they shared the prejudices of Southerners.
Also, freedmen were increasingly coming to participate in the new industrialized economy, which developed rapidly after the Civil War. This caused widespread resentment among white workers who felt they were being pushed out of their jobs by an influx of black labor. This fear was particularly acute in the Republican-dominated Northern states. Republican politicians soon realized that there were no longer any votes to be had in pursuing a radical civil rights agenda. As a result, they allowed their own legacy to wither and decay, resulting in serious consequences for successive generations of African Americans.
https://www.history.com/topics/black-history/black-codes