In regard to bluing tripods, bluing is a very poor choice for a finish that must be reliably weather resistant. It does not tolerate dampness well and does not provide a surface that is relatively impervious to wet. Bluing requires constant attention and maintainance to protect it from wet conditions or even mild dampness, and it is very poor in prolonged exposure without protection.
Bluing that is painted over generally wears away pretty quickly since it is not an especially practical surface for paint adhesion. However, bluing under paint, often does survive well, but all the blued finishes I have countered under paint, have been applied to lightly or heavily polished surfaces, which is not an especially good surface for paint adhesion. However, when there is a 100% chance that a tripod is going to be painted, I cannot see any rationale, for a number of reasons, for bluing it as a base after production. Primer, cadmium plating, phosphate get, and other more weather resistant finishes that are more adhesive than bluing to hold paint, but not bluing.
There are other reasons I can think of as to why a blued finish would not be used for tripods, but the above is the most important.
Currently I have examples of every type of vintage tripod issued with some exceptions that are jusr not available or were only made as prototypes or short runs. None were blued and I can't think of any vintage issue tripods that were blued for any vintage MGs going back to Maxim's early guns. Black oxide finish on tripods is fairly unique and recent such as for the PKMs. In my experience, T&E rear mounts, jam handles, chains, pins, traverse stops and similar small parts have been blued and sometimes painted, but that is about it.
I've owned probably thirty MG08 sleds of all makers, and still have fifteen, and have seen a lot more than that. Of course, that is not a statistically valid number to be representative of all sleds, but I believe it is adequate to determine this issue. None of these tripods showed any evidence of bluing as a factory finish that I can recall, and of the half dozen that I have restored, several with chemical paint removers, none showed any evidence of bluing as a base finish.
The possibility that areas of exposed rusted steel were originally blued on all these old sleds is extremely remote, and in my experience, not possible. The last point against the rusted steel areas having had bluing for a base finish is easily verified by looking at the bluing under the painted jackets and occasional receivers of other vintage MGs like Schwarzlose, Vickers, etc but mostly 0&s and 08/15s. Most of the time, the bluing is still there, protected by the paint from corrosion and rust. Even bluing under paint that has chipped or peeled, that has been exposed over many years to the elements can still be seen. If sleds had been blued there would still be lots of bluing remaining under coats of paint on all surfaces of the sleds, but I have never seen any such remains. Painting of the receivers and jackets occurred in all armies, but painting wasn't used to protect the finish, but for camouflage, with protection of the finish an added advantage. The MGs themselves would be well cared for, but mounts would need to be finished in a manner that would reduce maintainance as close to zero as possible.
So, I think it is fair to say that MGs with blued finishes would be subject to continuous and thorough cleaning, oiling and protection as much as humanly possible under extremely adverse conditions in wartime. Expending such effort on a blued mount doesn't make any sense to me when paint is the practical solution, so wasting time, expense and labor to blue mounts at the factory, only to require painting to protect them also doesn't make sense.
That's the best I can make it from my experience. FWIW