I married a southern gal, eastern North Carolina to be exact. As a Minnesotan, I have become familiar with the following:
1. "[he/she/they] are doing the lord's work" roughly translates to, "that task is so terrible/annoying, but someone has to do it, so props to them for doing it without complaining."
2. "bless their heart" roughly translates to, "they are really trying, but wow, so much fail"
As someone from NC with a LOT of family from eastern NC specifically, I'd like to provide an addendum to "bless your heart".
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.
1. "[he/she/they] are doing the lord's work" roughly translates to, "that task is so terrible/annoying, but someone has to do it, so props to them for doing it without complaining."
2. "bless their heart" roughly translates to, "they are really trying, but wow, so much fail"