If a "blue gun" means a gun that is safe to perform non-live fire drills, then it's just another term for an unloaded gun.
No a blue gun is just that, a blue gun, or a blue plastic gun that resembles a certain firearm. They're used for training so you don't have to use "unloaded" guns since and gun, whether you think it's unloaded or not, its treated as if it is loaded.
They're used for practice drawing, take aways, and retention.
It's not very smart to preach that all guns are loaded then turn around and use a real gun for such training.
Check out the video in the other post about "serpa's vs trigger fingers" to see what I talk about when I mention "blue guns".