As far as the word Zanj being equated with a slave, even among the Shia Muslims it referred to their ancestors which came from the region in East Africa that was known as the Land of Zanj, the Land of the Ancestors. It was a gold producing region which was and is still referred by some as Azania, which stretches from modern Somalia (The land of Punt) down to the Limopo River.
Several of the wives of the Shia Imams known as the "Twelvers" are from this region and are highly regarded for their scholarship.
Numerous examples, perhaps most notable is Hamidah Khatun.
http://www.al-islam.org/slavery/4.htm