The Low Country in South Carolina and adjoining states was once full of rice plantations, and enslaved people were brought in from from West Africa, particularly Sierra Leone, for their expertise in cultivating rice. Their descendants are the Gullah people, who preserved their languages and traditions brought from West Africa. Another crop that was concentrated in the Low Country is indigo, which produces a blue dye coveted around the world before synthetic blue dyes were developed. The blue color has spiritual meaning to the Gullah. The tradition of painting one's porch blue, particularly the ceiling, is to protect the houses from haints that would otherwise bedevil the inhabitants. The custom spread across the South when it became clear that "haint blue" also helped control insects. In the South, controlling insects is crucial. You'll still see porches with haint blue on the ceiling, because it's pretty and it's traditional, even though the paint no longer contains indigo nor lime.