I’m in my 50s, native Texan, and it’s been a dig pretty much as long as I can remember. My mom has a similar opinion.
It was seemingly Yankees or folks from out west who moved into the area who adopted it and used it otherwise.
Maybe in different States it has been used it differently though?
If a child stumps there toe
you hug them and say bless your heart
Someone dies and you say of the surviving wife “ bless her heart I cannot imagine what she’s going through”
That’s too examples off the top of my head
I’m only 7th gen white Mississippian my experience with southern vernacular could be suspect
That was sarcasm lol