I know a lot of slaves became Christian for a variety of reasons, but did any become Jewish? Did they stay Jewish? I don't think there were a lot of Jewish slave owners but there were some.
Another question would be whether the slaves of Jewish owners were treated any better or worse than their Christian counterparts.
Very little is known about your very interesting question. First of all, converting to Judaism is very hard and entails a lot of learning that requires literacy, which was not widespread among slaves. In fact at various times and in various places teaching slaves to read was prohibited.
Nevertheless, it seems that some slaves of Jewish owners considered themselves Jewish, even though they most probably did not receive formal religious teaching and they might not always have been entirely clear on the exact difference between Judaism and Christianity as the two faiths told many of the same stories. Here is the story of one such (ex-)slave:
Identified by no moniker other than "Paul the Jew," Paul had almost certainly been owned since at least the 1850s by one of the slaveholding Jewish families in South Carolina. Apparently, he identified himself as a Jew, and even considered Saturday, not Sunday, the Sabbath. During the war, he wound up in Virginia, where he mixed with a group of free blacks who ultimately convinced him to be baptized, on the grounds that "perpetuating his loyalty to the Jews was no more than perpetuating his loyalty to his owner." For Paul the Jew, freedom came to mean not only "Christian freedom" but also freedom from his owner to choose to be Christian.
Some slaves, however, continued identifying as Jewish after emancipation, referring to the story of Exodus and how they had been set free, just as the Jews had been set free from the bounds of Pharaoh.
Source: Clinton, Catherine, ed. Southern families at war: loyalty and conflict in the Civil War South. Oxford University Press, 2000.
I don't want to hijack this thread, but as a follow-up question: do we know if the Jewish slave owners followed the laws of Judaism regarding slaves? I'm pretty sure that in Judaism you have to treat a slave well and set them free after 7 years. My guess is no, but it would be interesting to find out.
To add-on: How many Jewish slaveholders were there? Would slave dealers be against the idea of selling their "wares" to Jews.