There have been more slaves through out history that were not of african descent. Just an avenue for special status.
For example, I have two daughters who wear makeup that has a sparkly effect to it. Thousand of Indian children are forced to dig mica daily to be used as a component in that makeup.
I have a lot of sporting and camping equipment, so I'm supporting forced labor in China. Some of my cotton clothing probably originated with forced child labor in Uzbek cotton fields.
On the continent of Africa, - I use electronics, including a smart phone, so I probably use Coltan (Columbite-tantalite) in those. Coltan is mined in the Democratic Republic of the Congo by workers forced to work by armed guards.
As I said before, global trade is one of the factors behind the number of people in slavery today.