Oh, "bless your heart" is the most comforting and loving phrase imaginable -- from someone who actually means it.
My parents lived in the south (where I grew up) and to this day I remember each and every "bless your heart" that I heard during their final illnesses. And thanks to my family's loving and wonderful friends, I heard it quite a lot. That phrase resonates with me and soothes an aching heart like nothing else.
What we are talking about is entirely different -- it's the "bless your heart" that precedes snark. I would call it a bastardization of a lovely bit of language.