Slavery started in 1619 and ended in 1865. In this period many black people worked without pay, for free. Also, pain and suffering was inflicted to them. For these reasons former slaves and descendants of slaves should have been paid reparations.
Slavery was horribly wrong and a horrible crime. Owning a person was a crime, is a crime and always will be a crime. Because it was written on a piece of paper that people could own people, does not mean that it was not a crime. It was a justification of the crime. This is called the Law of the Crime.
Source: Slavery-Crash Course U S History #13 (minute: 0.26 to 0.28)