A fascinating resource for this kind of question is "Born in Slavery: Slave Narratives from the Federal Writers' Project, 1936-1938", available at the Library of Congress. https://www.loc.gov/collections/slave-narratives-from-the-federal-writers-project-1936-to-1938/about-this-collection/

Peruse most any of the narratives, and food is often mentioned. Several mention very harsh conditions - I just skimmed one that said that children were only allowed cornbread and "pot-licker", which is the juice from stewed greens such as collard greens. This would have been full of vitamins and likely made with lard for fat but devoid of much protein. Another mentioned that Confederate soldiers raided a farm in Missouri and stole the meat from several hogs that were stored in the smokehouse, which the slaves had intended to eat themselves.

Other accounts indicated they were well fed, and some even say well treated (that is not the norm, from the small sample I have read). Typical fare would have been typical of a southern diet at the time - cornbread, rice, pork of some kind, biscuits or hardtack (a kind of bread), greens. One mentioned they had a garden that would grow "most anything you might see growed at the time [sic]"

Overall this is a fascinating narrative, and likely one of the most important original sources if you want to explore this kind of question in depth.