The term Bilad al-Zanj (lit. "Land of the Zanj") refers to the East African coast, stretching from Somalia south to Mozambique. The term did not include Ethiopia or Nubia, because there were separate Arab terms for those regions.

One very useful source that comes from the time slightly before the Zanj Revolt is Al Fakhtar al Sudan min al-Abayd by al-Jahiz. The work is essentially a rebuttal of 9th century Arab stereotypes about the people of East Africa.

Crucially, in one section al-Jahiz gives the following geographic indicators:

They say: You have never seen the genuine Zanj. You have only seen captives who came from the coasts and forests and valleys of Qanbuluh, from our menials, our lower orders, and our slaves. The people of Qanbaluh have neither beauty nor intelligence. Qanbaluh is the name of the place by which your ships anchor. The natives in the Bilad Zanj are in both Qambalu (Pemba) and Lunjuya (Unguja), just as Arabs are the descendants of Adnan and Qahtan in the Middle East. You have yet to see a member of the Langawiya kind, either from the coast (al-Sawahil), or from the interior (al-Jouf). If you would meet these, you would forget the issue of fair looks and perfection.

Qanbaluh/Qanbuluh/Qambalu are all a variety of spellings for the same location, on the island of Pemba off the northern coast of Tanzania. Lunjuya is even more recognizable as the island Unguja, also off the coast of Tanzania, to the south of Pemba.

Of course, the phrase "Qanbaluh is the name of the place by which your ships anchor" is key here. That indicates that the islands of Pemba and Unguja are merely trading entrepots, and that the origin of the slaves come from the coasts and inland adjacent to the islands. From these ports, and others, slaves were brought by ship to the Persian gulf region.