There’s always more to say, but as a starting point, you may be interested in this older thread:

• It is estimated that a minimum of 18 million Africans were enslaved by Arab slave traders. What ever happened to these slaves and why weren't the integrated into Arab society like they were in the Americas?, which mainly links to still older threads, including from myself (about Afro-Turks), /u/Commustar (about the Arab word), /u/sowser about important difference between the Arab and trans-Atlantic slave trades, and /u/captbuck about how differently racial lines worked in the Arab World.

In short, many assimilated (particularly the children of female slaves and their masters—these were not all sub-Saharan Africa slaves, either, c.f. the white Circassian slaves), many had no children (particularly the male slaves), and also some Black Muslim communities still do exist in the Middle East, for instance the Afro-Turks around the Turkish city of Izmir and the so-called “Al-Akhdam” in Yemen.

Many contributing factors.

• 1)Since Islam put laws in place that discouraged slavery outside of absolute necessity, and encouraged freeing slaves as an act of worship, seems like they were being freed/birthed into freedom at a high rate.

According to Bernard Lewis, the growth of internal slave populations through natural increase was insufficient to maintain slave numbers through to modern times, which contrasts markedly with rapidly rising slave populations in the New World. He writes that

a) Liberation by freemen of their own offspring born by slave mothers was "the primary drain,” and b) Liberation of slaves as an act of piety, was a contributing factor.

• 2. Many went to North Africa and mixed with local Arab/Bedouin/Berber/Amazigh populations. North African countries have significant populations of black/mixed people (especially Morocco, Algeria, Egypt).
• 3. Some didn't go anywhere: for example, 10% of Saudi Arabia's population is Afro-Arab, so there is a significant population of black people in the Arabian Peninsula today.
• 4. Many slaves of the Arabs weren't black.

For a long time, until the early 18th century, the Crimean Khanate maintained a massive slave trade with the Ottoman Empire and the Middle East. Between 1530 and 1780 there were almost certainly 1 million and quite possibly as many as 1.25 million white, European Christians enslaved by the Muslims of the Barbary Coast of North Africa

• 5. And of course, perhaps most significantly: They went back to where they were originally from. African slaves in Arabia could have been freed in many circumstances, and could have ridden horseback/caravan back to west/central Africa. That's a couple weeks on horseback.