This site has an incredible compilation of sources on medieval East Africa.
A 9th-century Description
This passage was written by the 9th-century litterateur, Jahiz. Jahiz was one of the most famous authors of the ‘Abbasid period, known for his sharp wit. He wrote on all sorts of topic, including a famous Book of Animals and another Book of Misers. Jahiz’s grandfather was reportedly Black, but he identified as an Arab.
Everybody knows that the Zanj are among the most generous of mortals; a quality that is found only among noble characters. These people have a natural talent for dancing to the rhythm of the tambourine, without needing to learn it. There are no better singers anywhere in the world, no people more polished and eloquent, and no people less given to insulting language. All other peoples in the world have their stammerers, those who have difficulty in pronouncing certain sounds, and those who cannot express themselves fluently or are downright tongue-tied, except the Zanj. Sometimes some of them recite before their ruler continuously from sunrise to sunset, without needing to turn round or pause in their flow. No other nation can surpass them in bodily strength and physical toughness. One of them will lift huge blocks and carry heavy loads that would be beyond the strength of most Bedouins or members of other races. They are courageous, energetic, and generous, which are the virtues of nobility, and also good-tempered and with little propensity to evil. They are always cheerful, smiling, and devoid of malice, which is a sign of noble character. Some people say that their generosity is due to their stupidity, shortsightedness and lack of foresight, but our reply is that this is a scurvy way of commending generosity and altruism. At that rate the wisest and most intelligent man would be the most stingy and ungenerous. But in fact the Slavs are more stingy than the Byzantines, and the latter more intelligent and thoughtful; according to our opponents’ argument, the Slavs ought to be more generous and open-handed than the Byzantines.
Likewise we see that women have less sense than man and children have less sense than women, but are meaner than they are. If more sense meant greater meanness, then the child should be the most generous of all. Yet in fact we know nothing on earth that is worth than a boy, for he is the most untruthful of mankind, the most calumnious, the nastiest, and the meanest, the least inclined to do good, and the most ruthless. Only gradually does the boy leave these qualities as he gains in sense and gains in good deeds. How then can the lack of sense be the cause of generosity in the Zanj? You have admitted that they are generous, and then you make assertions which are untenable, and we have already shown you the fallacy of your argument according to true reasoning. This opinion would mean that the coward is wiser than the brave man, the treacherous wiser than the loyal, and that the worrier is wiser than the patient man. This is something for which you have no proof. These qualities in man are a gift of God. Sense is a gift, and good character is a gift, and generosity and courage likewise.
The Zanj say to the Arabs: You are so ignorant that during the jahiliyya (the times of ignorance ) you regarded us as your equals when it came to marrying Arab women, but with the advent of the justice of Islam you decided this practice was bad. Yet the desert is full of Zanj married to Arab wives, and they have been princes and kings and have safeguarded your rights and sheltered you against your enemies.
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The Zanj also say : We also have philosophers from among us as well as theologians and we have fine manners. God may he be exalted , did not make them black in order to disfigure them; rather it is their environment that made them so. The best evidence of this is that there are black tribes among the Arabs, such as the Banu Sulaim bin Mansur, and that all the peoples settled in the Harra, besides the Banu Sulaim are black. These tribes take slaves from among the Ashban to mend their flocks and for irrigation work, manual labor, and domestic service, and they take their wives from among the Byzantines; and yet it takes less than three generations for the Harra region to give them all the complexion of the Banu Sulaim. This region is such that the gazelles, ostriches, insects, wolves, foxes, sheep, asses, horses and birds that live there are all black. White and black are the results of environment, the natural properties of water and soil, distance from the sun, and intensity of heat. There is no question of metamorphosis, or of punishment, disfigurement or favor meted out by Allah. Besides, the land of the Banu Sulaim has much in common with the land of the Turks, where the camels, beasts of burden, and everything belonging to these people is similar in appearance: everything of theirs has a Turkish look.
A 10th-century Description
Istakhri was an Iranian geographer who wrote his geographical text about the Islamic world. He did not include the Zanj, but the region appears labeled on his maps. He also includes an explanation for excluding a broader discussion about the Zanj.
Here we shall not mention such peoples of the Maghrib as the Blacks (Sudan), the Buja and the Zanj, because, whereas the sound political organization is based on good morals, religion and permanent institutions, these peoples possess none of these. Only a few branches of the Blacks who dwell along the borders of the aforesaid empires [i.e. the Byzantine and the Moslem empires] possess some religious beliefs, moral and political institutions similar to [those of] the above mentioned empires. Such are the Nubians (an-Nuba) and the Ethiopians (Ḥabasha) because they are Christians and follow the customs of the Rum. Before Islam, those peoples had some links with the empire of the Rum because they were neighbours. In fact, the territory of the Nubians borders on Egypt and that of the Ethiopians (Ḥabasha) faces the Red Sea. Although the Ethiopians are separated from Egypt by a desert – where gold mines are found – they are in touch directly with Egypt and Syria (ash-Sham) through the Red Sea. Nothing farther can be said in their favour.…………………..
The inhabitants of the south are of a dark complexion, and the blackness of their skins increases as they dwell farther to the south.
You can see the Nile (a straight vertical line) with Sudan to the right and the Maghrib (North Africa under it). Off the Nile is “al-Bajja,” then to the left is the Zanj.
A 13th-century Map
A 13th-century Depiction
This image of a market comes from a book of stories by Hariri, copied in the 13th century.
Medieval Islamic History 