People’s looks are often mentioned with slavery, though. For example, while Julius Caesar abducted about a million Gauls, the blondest slaves in Roman markets were Angles.

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.