I didn't think it was commonplace to have slaves from Africa, if in fact that is where Samson derives from. Where did the caravan come from?

Slavery was not a race-based practice in the Roman Empire. Slaves could be of any ethnicity or nationality, and possibly as much as one-third of the empire's population was made up of slaves. With that in mind, it's not unusual that Samson, the large and powerful deaf man that Rosh determines to capture, would be a black slave. Samson could have come from Egypt. In 30 BCE, after Cleopatra VII died, Rome annexed Egypt as a province, so people from Egypt could well have migrated or been brought to Palestine in the first century CE. Even before that, Rome had taken control of northern Africa along the Mediterranean coast. As far back as the First and Second Punic Wars, Rome had won battle victories in Africa and could have taken black Africans captive as slaves. Samson could have been a descendant of black people captured from Africa two centuries prior to the events of The Bronze Bow, or he could have been captured in some relatively recent skirmish in Egypt or northern Africa. Within the empire, some parents even sold their own children into slavery to help alleviate their family's poverty. Due to the widespread use of slaves in the Roman Empire, it would not be at all surprising to see a black slave or a slave of any race or ethnicity around the time of Christ.
https://www.cemml.colostate.edu/cultural/09476/egypt02-07enl.html