Shelby Starnes and John Meadows host Blue Collar Muscle. What makes them blue collar. I also heard Evan Centopani is a blue collar bodybuilder. Usually those who say they are blue collar are from big cities and say they got their work ethic to go to the gym and diet from their respective environments and that they mean business and take no crap.
Not sure what Shelby did for a living before bodybuilding, but John was a project manager at Chase. Far from blue collar.
I think what "blue collar bodybuilding" means guys who depend more on heavy lifting/hard training and drugs vs the genetically gifted like Phil or Dex who can walk into a gym, get a quick pump, and consider it a workout.
Someone like Dorian comes to mind. He used to grind it out at the gym until muscles tore. Branch falls into that category, too.
Guys like Cutler, Gunther, Markus, Dexter, Phil, etc.. would be considered "white collar bodybuilders."
So, as you said, no thing to do with where they grew up or what they do for a living, but how they conduct themselves in the gym.