I missed out on the "Bless Your Heart" thread you're talking about.
I do know one thing, though. If you hear anyone preface a remark by the phrase, "Bless her/(his) heart", get ready...the rest of the sentence is not going to be complimentary, nine times out of ten.
There are times when the "bless your heart" is sincere, but for the most part, it is the standard preface used when one wants to be critical and bitchy, while at the same time trying to appear charitable.
To whit:
(The catty remark:)
"Bless her heart, she just can't keep that weight off, can she?"
(The backhanded compliment)
"Bless his heart, he ain't got a lick of sense, but he works hard."