I think that most white Americans either actively persecuted blacks or just simply didn't care about what was happening to them. That's why segregation existed in several states. The Southern states were one group. North Dakota, Kansas, South Dakota, Utah, Oregon were others. I really think most whites were uncaring.
Firstly, you say white Americans as if all white Americans were racist, end of story. That is untrue and we can clearly see it throughout the civil rights campaign. Often, white people would walk at the front of black protests to prevent police intervention, as many courts would be fair if the white man sued the police officer on assault. Now, I'm not saying racism didn't exist, but we can clearly see that some people cared strongly and (many famous cases) got arrested for it.
Secondly, you have to look at the idea of social "normality" and how it can be manipulated. In the same way that we don't question, that swearing is socially wrong etc., back then it was conditioned into people from a young age that black people were an inferior form of human. This is rooted from the age of slavery where the slaves were treated as animals; the beliefs get passed and it's hard to remove. We can even still see that today with racism still lingering. But I would be wrong to say that people didn't care. In the times of slavery maybe, but when the slaves won freedom, many people adapted to the idea it was another race of human, but just lower standard. Of course there are exceptions of extreme racists but overall, people did care but just not as strongly as they did about white people.