I feel like it's also context dependent.
"You got me Swedish candy? Bless your heart!"
"Oh, Philip got arrested again? Bless his heart."
it doesn't matter the context, it's adding in the "little ol' " part to it that makes it a dis vs really blessing the person and hoping things turn out better for them. "oh, Philip got arrested again? Bless his heart" Pretty much is asking a higher being to bless him and teach him the right way to do things, Not dissing. Dissing = "Oh, Philip got arrested again? Bless his little ol' heart" an even bigger diss = "Oh, Philip got arrested again? Bless his cott'n pick'n little ol' heart" That one right there is when you just clam up and walk away. Cause that person is mad in more ways than one.
just a little heads up. "bless your heart" is a really good meaning, it does mean bless your heart as a thank you. NOT a diss. If they say "Bless your little ol' heart" THEN it's an insult. there is a difference in the south about the two. i have many, many, MANY, southern friends, and family (mothers side) down in the south. Please don't get them mixed up. you'll insult someone without meaning to and give thanks to someone when you really mean to insult them.