My grandmother was from NC, but I still don’t have a handle on “bless your heart.”
Sounds like your granny was a believer. That is normally what a nice little old lady that believes says when she thinks kindly of you and/or whatever it is that might be bothering you. I.E. you fell and skinned your knee. She would say that to make you feel better. It is just a way of saying May GOD bless you and your hearth.