I suppose that some things (dogs included) might be called "blue" in reference to their perceived color, but in back country Florida, color is irrelevant. Folks lovingly refer to dogs, horses, rifles, shotguns, trucks, friends, even spouses, as "Ol' Blue." The usage usually implies a long-standing relationship with the thing or person, and a substantial sense of reliability and constancy, or perhaps just familiar predictability. I'd say the term is mostly one of respectful endearment.