the first people of iran were the elamites,which have been described by many people asnegriod,but probally related tothe dravidians in india.
the people you refer to in southern iran are probally desedants of slaves,but many irian scholars have sugested they were the orgiinal people of iran which were called ugly the the invading indo iranian persians.
Not all of them came as slaves form the portugeese some came on their own free will and settled on their own. there is a settlment of ex slaves which was probally brought there by persians caleld zanjian,which litteraly means village of africans.
the iranian goverment neglects these people in iran,which shows that racism even exists inthe so called islamic world.