It is common to call the enslavement of black Africans “America’s original sin”—the uniquely devastating evil from which all American failures have flowed. In fact, American slavery was benign compared to the much more extensive, vastly crueler practice of slavery in the Middle East.
Because there are so few people in the region with black features, it would be plausible to assume that hardly any black slaves were brought in. Nothing could be further from the truth.
The United States abolished the importation of slaves in 1808. Before that, 361,000 African slaves had been imported into English-speaking North America.  Through natural increase, their number in the United States rose to 1.75 million in 1825 and 4.5 million in 1861. By contrast, from the time of the Arab conquest of the Middle East in the seventh century, approximately 14 million black slaves were imported into the area that extends from Morocco through Iran. 
The first reliable census conducted in the Middle East was in Egypt in 1798. It recorded a total population of 2,400,000.  When reliable population data first became available for the entire Middle East, Egypt accounted for about a quarter of the region’s population. So in 1800, the population of the area from Morocco through Iran was probably about 10 million.
Why had the Middle East not become overwhelmingly black and mulatto?
One reason was extremely high slave mortality. As the Encyclopedia of Islam (second edition, 1960, volume I, page 36) notes, “The high mortality rate which overtook these coloured men in Persia prevented them forming an important element of the population.” I will provide two examples of this—one from North Africa, the other from Iraq: “Of the Saharan salt mines it is said that no slave lived for more than five years;” and “the black slave gangs that toiled in the salt flats of Basra [Iraq] . . . numbering some tens of thousands . . . were fed, we are told, on a few handfuls of flour, semolina, and dates.” 
In addition, casual mating was not permitted and marriage was discouraged. Consequently, of the 3,000 female slaves emancipated in Zanzibar in 1860, only five percent had ever had a child.  Many of the children born to slave women were murdered. In 1856, the Anti-Slave Reporter observed that in Constantinople, the murder of the babies of black slave women was practiced “as a matter of course and without the least remorse.” As a result, in Constantinople, “it was commonplace for Turkish gentlemen to have numerous [black] concubines, [but] it was rare to see a mulatto.” 
As for slave men, many were castrated. Castration was lethal for the large majority of slaves on whom it was inflicted, especially blacks. White eunuchs were produced by merely cutting off their testicles, but blacks were subjected “to the most radical form of castration . . . level with the abdomen . . . based on the assumption that blacks had an ungovernable sexual appetite;” “every [black] eunuch represented at the very least 200 Sudanese done to death;” and at the beginning of the tenth century the caliph of Baghdad alone had 7,000 black eunuchs. 
Approximately five percent of the maternal ancestry of Middle Eastern Arabs is African (as determined by mitochondrial DNA), but nearly none of their paternal ancestry (as determined by Y chromosomes).  So, some of the offspring of mating between Arab men and black slave women survived, but nearly no offspring of black slave men.
This disregard for the lives of black slaves reflected their low value. In Arab and Persian armies, white slaves—Turks, Slavs, Berbers, Kurds—were the cavalry; black slaves were occasionally infantry but more often did menial non-military work. Barracks and residences were racially segregated, and white soldiers sometimes slaughtered blacks for behaving disrespectfully. Among domestic slave women, blacks performed menial work while whites tended to be personal attendants of their owners. White slave women cost between three and ten times more than brown-skinned Abyssinian women; black slave women cost half to one-third the cost of brown slave women.  In 1825 a British traveler in Egypt wrote, “It is the fashion here . . . to consider the Negroes as the last link in the chain of humanity, between the monkey tribe and man in intellect.” In fact, “To the present day, in North Africa, a man with Negroid features, even of the highest social status, is sometimes described as ould khadem, ‘the son of a slave woman’;” and “In Arabia even a pariah tribe like Hutaym disdains miscegenation.” 
Middle Eastern slavery was horrific not only because of its savagery, but also because it persisted so long. Journalist John Laffin recorded a slave auction he attended in 1956 in Djibouti, to which Arab slavers had brought black captives. Dealers from Arabia bought them for the slave markets of Jedda and Medina.
Men, women and children were brought from the warehouse and paraded on a raised platform . . . . A trader would nudge a slave’s jaw with a stick and the man would open his mouth to display his teeth. Another probe with his stick and he would flex his muscles. Young women were forced to expose their breasts and buttocks. A dispute arose over the virginity of a tall young ebony woman and during an hour-long argument she was forced to squat while one of the most prominent buyers examined her with his fingers. She was terrified; her trembling was visible fifty yards away. Occasionally children were sold in batches. . . . [T]hey held tightly to one another and kept looking around as if for help. Boys of about ten or twelve had their anuses examined; homosexual buyers are fussy about disease. . . . [P]erhaps 200 slaves changed hands while I was present. 
Black slaves continued to be imported into the Arabian Peninsula into the 1970s, since slavery remained legal until then in the Sultanates of Muscat and Oman.
There are still slaves in some countries, most notoriously Mauretania, which did not abolish slavery until 1981 and made it a crime only in 2007. And yet, somehow, it is the United States that is said to be permanently scarred by a practice that ended a century and a half ago.
Braudel, Fernand 1981: The Structure of Everyday Life.
Cochran, Gregory and Harpending, Henry 2009: The 10,000 Year explosion: How Civilization Accelerated Human Evolution.
Eltis David 2001: “The Volume and Structure of the Transatlantic Slave Trade: A Reassessment.” William and Mary Quarterly, 3rd series, 68,1: 17-46
Gordon, Murray 1989: Slavery in the Arab World.
Laffin, John 1982: The Arabs as Master Slavers.
Lewis, Bernard 1990: Race and Slavery in the Middle East: An Historical Enquiry.
Segal, Ronald 2001: Islam’s Black Slaves: The Other Black Diaspora,
Sheriff, Abdul 1987: Slaves, Spices and Ivory in Zanzibar: Integration of an East African Commercial Empire into the World Economy,
 Eltis 2001: 45. I cite references by author’s last name, date, and page number. Full information is available in the bibliography.
 Segal 2001: 55-61, 147.
 Braudel 1981: 43.
 Lewis 1990: 14, 56.
 Sheriff 1987: 59.
 Gordon 1989: 17.
 Segal 2001: 41, 52, 156.
 Cochran and Harpending 2009: 153.
 Gordon 1989: 56-7, 65-75, 81-2, 98-103; Lewis 1990: 56-9, 65-9, 74-5; Segal 2001: 46-9.
 Lewis 1990: 70, 90-91.
 Laffin 1982: 4-5.