I think pretty well all scopes were blued as they left the factory, apart from some Mk3's (of Taylor Hobson manufacture. I'm not sure, & someone can possibly chip in on this, but some late Kershaw Mk3's may also have had a chemical finish). However, most Mk3's & I think (sticking my neck out) pretty well all earlier scopes, were blued originally. Many have been refinished in military service, as well as by civilians, since.
You will, however, see black paint on earlier scopes on the brass parts, as of course, conventional bluing techniques don't work on brass. I think some manufacturers chemically blackened the brass larger components but the majority just used black paint.