Mississippi Department of Archives
The MDAH in Jackson holds plantation records, Freedmen's Bureau correspondence, and county chancery court records dating to the 1830s.
Freedmen's Bureau Mississippi records
Searchable at FamilySearch (free with account). Marriage contracts, labor contracts, and complaint records are gold for tracing ancestors through the 1865–1872 window.
1870 + 1880 census
First two census years with Black names. Mississippi census records are well-preserved. Cross-reference with the 1860 Slave Schedules to identify slaveholders.
DNA matching
AncestryDNA and 23andMe both have strong Mississippi-Black-American match databases. Connect with cousins you don't know and ask what they've found.