Doctor Who is a story about time travel, and he goes into Earth's past a lot. In Earth's past, there was a lot of blatant racism. It's only by acknowledging and showing how bad things actually were that you can highlight how far we have come in setting that right.
Black people weren't the only slaves in our past. For many years, Vikings raided coastal villages all around the British Isles and beyond, taking white people and selling them as slaves.
If we ignore history, we are doomed to repeat it. Many of us even now are treated like and feel like slaves just trying to make ends meet. Sure, we have the luxury nowadays of locking ourselves in our comfy cages at night, but we still lock ourselves in, and we still don't have the freedom to wander off and do as we please, if we have the money to do it we can't, we have to be at work, and if we don't have a job to be a slave to, we don't have the money to do what we want.