I see it frequently cited as always meant sarcastically/disingenuosly. It certainly can mean "screw you", "go away", or "... so much fail".

However, if there's one thing people should understand about southern/NC etiquette, it's that passive aggression is the primary form of aggression. There are plenty of southerners who would never tell you to GFY straight to your face. That doesn't mean they aren't thinking that and trying to say that, though. Perhaps even with a "bless your heart".

Given all of that, "bless your[/their] heart" is absolutely to be taken at face value about as often as it shouldn't. That level of plausible deniability provides the highest level of potential passive-aggressiveness.

I can't speak for absolutely everybody, but at least if someone from eastern NC says "bless your heart", they could mean anything between "GFY" and "I'm so sorry that happened, please come to my house so that I can shower you with hospitality". You might never know which they meant, and that's intentional.

For example: There's a fluorescent light above my desk that flickers and has been driving me crazy for years. I can't get facilities to fix it because they need a ticket that's approved by management and has an approved budget, and all their other tasks are more urgent.... so it slowly drives me mad for a year. Finally, a facilities person says "You know what... I'm not supposed to do this without management approval... but I'll just fix this real quick and it'll be our secret" ..... Me: "Oh, thank you, you're doing God's work"