As noted, rust bluing is a form of rust, and several gunsmithing books I've read caution against using WD-40 or any so-called polarizing or rust penetrating oil initially. They say the fresh black oxide is not tightly bound at first and these types of oils will remove some of the new finish. I use plain old Outers Gun Oil on a freshly rust blued barrel. I've come to like Ballistol, and have had no problems with it. I do like to use WD-40 with 0000 steel wool to clean a neglected gun as it seems to loosen any fine dry rust better than many other cleaners or oils, and still leaves the tightly bound patina. I agree that lemon oil is probably slightly acidic, and it might be interesting to drop a blued screw into a small jar of it to see if it might actually remove bluing.
