Wiping my lugers

[Major Tom]

Every so often I take mt lugers out of the safe and wipe them down with a slightly oily rag. I've noticed that after doing this there are rust colored spots on the rag like I'm wiping off rust. I've been doing this for years with same results. Pistols do not show any rust on the finish. So, is this a result of the rust blueing they used back then? Finish, BTW, is not getting thinner.