The concept of "racial superiority" predates Darwin by decades, if not centuries. The Spartans believed in the concept, for example. The domination of blacks by whites in the United States prior to the War Between the States can be chalked up to specific Christian doctrines -- blacks were considered "the children of Ham."
And Ham, the father of Canaan, saw the nakedness of his father, and told his two brethren without. And Shem and Japheth took a garment, and laid [it] upon both their shoulders, and went backward, and covered the nakedness of their father; and their faces [were] backward, and they saw not their father's nakedness. And Noah awoke from his wine, and knew what his younger son had done unto him. And he said, Cursed [be] Canaan; a servant of servants shall he be unto his brethren. And he said, Blessed [be] the LORD God of Shem; and Canaan shall be his servant. God shall enlarge Japheth, and he shall dwell in the tents of Shem; and Canaan shall be his servant. [Gen 9:22-27]
Agreed. However, it was Darwin who supplied the scientific basis from which eugenicists and racists of various stripes were able to make "objective" claims.
One need not (and I don't) ignore the fact that people used Biblical claims in defense of slavery. However, Mississippi's "imperious law of nature" looks to be based more on evolutionary claims, than on Biblical ones.