I was at the library today, we have many of the published census indexes.I found Reuben Bull, 1850 census, Yazoo Co, MS, page 508.So, now you have a county in MS to research.(^_^)
I recommend you check out the Yazoo Co, MS GenWeb site.Click on Surnames and Researchers, there are 3 people listed as working on Bull family.I would contact each of them, see if they know anything about Reuben.
Story of Black Jack Bethel Church was submitted by Roger Bull, he has posted lots of queries and responses, so very active in his research.I just did a quick scan, print not real clear, but looked interesting.From church minutes it looks like they allowed black slaves to attend the church.As they were baptised they were listed by name and who owned them.
I think you're going to have some fun with Yazoo Co, MS records.(^_^)