I bless hearts like that too. It's a southern thing. I on rare occasion will say "bless their hearts" for the irony of it, or to be a smart@ss, but for the most part when I say it I mean it as a blessing.My grandmother was a sweet old southern woman. No matter who she was talking about, good or bad, all she ever said was "Bless heart." Not his or her, just heart. Sometimes it was an expression of gratitude. Sometimes it was wishful thinking for divinity to touch whoever she thought needed saving. Either way, it was genuine.
And I know I'm old because I've started saying it.
*sigh*
I like how your grandmother didn't differentiate, but just said "bless heart," that makes it a more general blessing.