If they had a kind master they were sometimes allowed to learn how to read and write

This statement is wildly misleading and sometimes is doing a lot of work. I'll defer to those who study the history of the "Arab world" regarding the second part of your question (and the inaccuracies of that post) but can unequivocally say learning to read and write was rarely, if ever, used as a reward in the system of chattel slavery. Instead, enslaved people would be taught to read and write if their owner felt that knowledge would explicitly benefit the owner or the owner's family. There are isolated instances - most notably Frederick Douglass, who was taught by his owner's wife - but even then, the person making the choice to teach an enslaved person was most likely breaking the law.

Anti-literacy laws for enslaved people as well as freed Black adults and children were the norm across the Southern states as slave owners saw a relationship between literacy and resistance. To be clear, this didn't mean enslaved people didn't find ways to educate themselves and their children. There's clear evidence they did as states kept having to revise anti-literacy laws with stricter and stricter penalties including corporal punishment and in some cases, death. The punishment, in many cases, extended to those who taught the enslaved person to read. The motivation behind those laws were often freed Black educators and white abolitionists who helped lead or facilitate clandestine schools that met before dawn or late into the night. In some cases, especially Louisiana, even being caught with literacy materials that might be used to educate an enslaved person could be severely punished.

Finally, to paraphrase /u/freedmenspatrol, there was no such thing as "kind" master. Not a single aspect of chattel slavery can be described as kind. In addition, the history of Black Americans who owned slaves themselves is complicated and often tied up in the history of previously enslaved people purchasing members of their own family.