Can I ask where you are deriving this figure from?

This looks like a combination of the estimates of the Trans-Saharan Slave trade plus the trade originating in East Africa and proceeding into the Indian and Atlantic Oceans (Alpers, Ivory and Slaves; Austen "Trans-Saharan Slave Trade" in Uncommon Market edited by Gemery and Hogendorn; Austen, "The Mediterranean Islamic Slave Trade" Slavery & Abolition, 1992).

Last year, u/Sowser wrote a rather amazing series of posts about a closely related question. Be sure to read on past the first reply, a lot of in-depth explanation follows.

These posts mostly talk about American slavery, which that user specialises in, but it illustrates that it's actually the United States that are exceptional in getting such a large black population out of a relatively small initial amount of slaves, and explains why. Later posts address circumstances in other parts of the Americas.

Further down that same thread, u/Commustar addresses the other half of your question, though not in quite as much depth. They describe where these slaves went (i.e. many never went to the Arab world as such) and how they were assimilated, whilst u/yodatsracist gives an example of a group of people of African descent that DO exist in the middle east, but just aren't very well known.

Through all this, you'll find that part of the answer actually is that these slaves were much more integrated in Arab society, to the point where they didn't continue to exist as a separate group with a separate identity in the same way black people do in America.

You'll find it well worth a read. One of AskHistorians' finest discussions.

I'm as pessimistic about your numbers as /r/RTarcher. Mostly a matter of logistics and usage. Studies on the trans-Saharan trade indicate that its main trade items were goods easily packed on camels, like salt and gold. Certainly slaves were not sold in large enough numbers to have drawn significant attention from the writers of the time. For comparison, the Sudan was dubbed by the Arabs, "The land of Gold" as opposed to say the Slave Coast. Please note, this holds only for the Sudan i.e West Africa, the Eastern Coast was called Zanj-bar, although to my knowledge I do not know if the argument on whether the coast informs the name given to the slaves of the famous Zanj revolt has been resolved. Sadly foreign historians+history students are often more informed on recent scholarship.

On the subject of usage, this often gets lost in the discussion is that the African states themselves used slave labour, this is recorded across the Sudan. Of course this always invites those creepy psuedo-justifications of slavery, but it remains uncontested. My point however is that most of the slaves involved in the trans-Saharan trade did not end up in Arab hands. For example, the Oyo empire's military superiority was based in some part on its calvary, purchased with slaves. However that trade flowed both wars, the slaves traded for horses ended up in the farms of the Sarkins of the North while those traded to the Oyo were prized for their horse-tending skills. In its own way, it was a variant on the Savannah-Forest trade based on comparative advantage.

So, recapping, your numbers are contested. It is simply inconceivable that such slow moving valuable cargo would have been profitable across the Sahara which lest we forget, as recorded by Ibn Batuta and tales of constant frontier wars with the Desert nomads, remained dangerous to caravans. The majority of the slaves of the trans-Saharan trade likely ended up in the salt mines and farms of the Sudan.

So now, to the meat of your question. First we know that black slaves were a factor in Arab society, words like Zanj and Abeed help us term this a certainty.

However, they seemed to have been a reduction in the import of slaves for labour, an aftereffect of the Zanj revolt against the Abassids.

That however would not be enough to explain the absence of slave descendants, succinctly, it would be the characteristics of historical Arab slavery.

The Romans were noted for manumission, the Americans for chattel slavery, in that vein the characteristics of Arab slavery would be, castration, slave soldiers and the harem system.

Castration and soldiery not lending themselves to descendants seems self evident. The harem system though might. You see African-Americans are an anomaly brought about by a unique American quirk, the "one drop rule" and the American fear of "miscegenation". In cultures less concerned about inter-breeding a distinct culture like the African-American one would have failed to emerge. That North America despite having received the least amount of African slaves has such a large grouping of their descendants was a result of deliberate policy. The Arabs bound by the strictures of Islam did not have the same problem of assimilation. For the descendants of those who could afford harems, their genetic history ought to reveal their ancestry.

Sources for the curious:

• On the Trans-Saharan Trade: The Cambridge History of Africa and the UNESCO series.

• On the presence of slaves within Africa: UNESCO General History of Africa.

• On Slave Soldiers and the Arab world: The Origins of Political Order

There's a lot of really good answers here and links to previous answers all of which are excellent. But to expand on /u/commustar and /u/yodatsracist's answers in this thread I just want to comment on:

why don't you see their descendants in Arabia today like we see African-Americans in the Americas?

Other than their points on integration due to a lack of prejudice against, or even lack of a concept of "miscegenation", you absolutely do see people who are very clearly of African descent in Arabia. This is perhaps most prominent in Oman, which controlled Zanzibar and the Swahili coast and was a major player in the Arab slave trade with Africa until it was de facto abolished by the British. It does not take very long in Oman to see the very large number of Omanis (with full citizenship, etc.) who are clearly of east African descent. Indeed a very large number of Omanis are, to this day, bi-lingual in Swahili and Omanis are still very prominent in the economic and cultural life of, say, Mombasa, Kenya, although they were largely killed or expelled from Zanzibar.

Less happily there is also a much discriminated-against group of African descent in Yemen.

It's also potentially worth noting that many east Africans, in terms of features like skin color or what have you, are less distinctive in appearance to begin with from many Arabs than they would be with West Africans. So you might not necessarily be able to tell apart a Yemeni, an Omani and, say, an Eritrean.

This is tangential but related- there actually is an African ethnic group of around 50-60 thousand in the Indian subcontinent known as the Siddis who are descended (in part) from slave populations brought in by Arab traders.