Serfdom is, of course, different from slavery. Thr main difference being that serfs could be sold like other mobilr assets as they were attached to the land. When thr land was sold, the serfs eere sold along with it so. The common theme between slavery and servedom so is that in both cases people were property of sorts.

Abolitionists were Christians, because they tried to abolish slavery in Christian nations. As much as Abolitionists used religious arguments against slavery, slavery advocates used religious argments for it.

Modern day de-facto slavery, in the Arab world and illegal varieties affecting immigrants for example, are different from historical slavery. As much different as Roman and Greek slavery was from Barbary state slavery and Colonial slavery.

One hallmark of the modern variety of slavery, the one European nations used to build their colonial empires on, is the racist nature of it. In ancient times race didn't play much of a role in slavery. Nor did religion in pre-Christian Rome. These two things set the Colonial / Christian form slavery apart from those ancient forms.

Hollywood, at least in the HBO production “Rome” did get this part right. The main character has blonde hair, which is a political liability.

As noted in another comment, Zanj is the Arab word for black. Kafir is the word for nonbeliever and use more broadly. I’ve not heard it used racially.

There wasn’t a law or practice of enslaving people based on their skin color. As evidenced by the free blacks in other areas of the world that weren’t enslaved.

I think the racial stereotypes came about based on the race of the slaves, not the other way around.

Just because that's all you know about doesn't mean that's all there was.