Oz:
My recommendation would be to rust-blue in the normal manner, boiling after the first and maybe second rustings, then simply carding without boiling for the last couple of 'rusts' (depending on your rusting formula).

This should give a good approximation of a rust-blued finish turning brown, but is subject to variation due to different types of steel and therefore requires some trial and error.

I once read of a gun-forger in the US who would spend hours in front of the telly in the evening bashing up the metal-work with an old bunch of keys to give it the proper aged patina!

I can imagine the look on your local pharmacist's face when you tossed that shopping-list on the shop-counter!