Going off the amano chibi sketch I would say he's of a descent which has naturally quite dark skin. Whether this is equivalent to a real-world African country or Australia (full blood Aboriginal people can be darker than a lot of african people btw) is uncertain, but I don't think it needs to reflect anything too specific. "Black" doesn't necessarily denote him being of a specific race, more a general (pun intended) "he's not caucasian".