Before hot caustic blue came into general use in the 1930's the primary blueing method was either heat or rust (called browning originally) blueing. Colt used the heat blueing method, Winchester used a mix of heat and rust blueing, with rust blueing being used on barrels and magazine tubes, and DWM that build the first Lugers used rust blueing.
To rust blue you coat the pistol parts with a corrosive mixture containing nitric acid. The parts are put in a humidity cabinet to speed up the rusting process, taken out and put in boiling water to stop the rusting. Then the rust is removed with what? Steel wool. When the rust is carded off with the steel wool it leaves a blue finish, and the process is repeated until the desired blue color is achieved. The steel wool removes the rust, but does not remove the blue.
You can rub the blue off a gun with most anything if you are not careful. You will see some autos with the front strap blue worn in the pattern of someone's fingers. Whatever you use, just use it carefully. The rust itself is the most abrasive, and even if you are using a soft piece of cloth, turn it frequently so that the rust doesn't scratch the glue. Bronze wool was developed for marine use, where the tiny steel wool fibers left in the wood of boats would rust.
This is a 113 year old DWM Luger in the original rust blue finish. It was scrubbed with steel wool repeatedly to achieve this blue finish.