While I’m not a southerner, I’ve lived in south off and on. I’ve always heard it in south, never in north. Endearing I suppose to shower with cuddles, attention, gifts .... one or more.
Now Bless Your Heart is straight up snark. While the degree can vary from “poor thing tried” all the way to “he doesn’t know what he doesn’t know” ... I would never use unless snark intended.
In south both very old sayings. Never heard either up north.