The trans-Saharan slave trade transported 11-17 million Sub-Saharan Africans to Islamic lands further north, yet none of these countries have a black population of descendants of slaves like the US does. What happened to them?
EDIT: So i found a book in French that sets out to answer this specific question: the veiled genocide by Tidiane N’diaye. Going of summaries I found online, black slave ‘reproduction’ was tightly controlled in Muslim lands (relative to western colonies where slaves forming families was more tolerated). This ‘control’ was done mostly through castration of males (of which only 30% survived the process). Women condemned to sexual slavery or house service, on the other hand, were subjected to such horrifying conditions that any children they had died young.
I can recommend some earlier answers on this topic. It seems most historians on this sub do not agree with N'diaye's ideas as you have presented them here.
One important difference which u/sowser points out here is that the ideology of racism made American slavery rather special historically, and helped keep the descendants of the enslaved separate. Also as u/yodatsracist has noted whilst linking to yet another discussion, there are Black communities in several Muslim countries dating back to the days of the slave trade
Furthermore, our eunuch expert u/caffarelli has discussed castration in the Muslim world here and here, and Islamic slavery in general here. The latter has also been described by u/Zooasaurus in this answer
TL;DR: Based on what we know about attitudes toward sexual relations with slaves in the medieval Middle East, combined with genetic evidence that seems to strongly point in that direction, the descendants of Black female slaves likely assimilated into broader society. As for Black male slaves, my understanding is that many were castrated and/or worked in working conditions with high mortality rates, but I’m not as knowledgable on that. Thankfully, however, the previous commenter kindly linked resources to more info on castration in the Islamic world.
N’diaye’s effectiveness at proving his thesis is questionable: https://en.qantara.de/content/tidiane-ndiayes-the-veiled-genocide-selective-theses-on-the-arab-slave-trade
Regarding your question however, there is a good book largely discussing Black slaves in the Islamic world about this issue that I recommend: John Hunwick and Eve Toutt-Powell, The African Diaspora in the Mediterranean Lands of Islam
It should also be considered that there is genetic evidence for the theory that the children of enslaved women assimilated into broader society e.g. modern Egyptians have a significant matrilineal Sub-Saharan African genetic component that is not found in DNA from pre-Islamic Egyptians (https://www.nature.com/articles/ncomms15694; https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1180338/). It’s a similar phenomenon for Muslims in the Levant on a south-north cline (https://www.haaretz.com/archaeology/2021-02-27/ty-article-magazine/.highlight/the-enigmatic-genetic-footprint-of-palestine/0000017f-ef3f-d8a1-a5ff-ffbf3de50000).
Anyway, historically, this genetic evidence would make sense. In most societies, the child of an enslaved woman was also considered a slave (Cambridge History of Slavery). But one of the unique exceptions was in the Islamic world where any son or daughter that a free man has with his female slave was born free, and there was not much stigma attached to being born of an enslaved woman versus a free woman. This then makes it easier for the enslaved woman’s child to then assimilate into broader society.