This might help. I have not tried this but it might work. If you rust the hammer and them wipe it over with a weak solution of Phosphoric Acid (its used in kurust) the rust will turn black. To keep the rusting process going the part needs to be kept damp. If it dries out the rusting stops.

BROWNING BARREL & OTHER PARTS.

Iron and steel parts such as the barrel, lock, trigger and screws should be browned to be authentic. This is accomplished by causing a controlled rusting to take place on these parts.
The metal should be polished sufficiently to remove rust, pits and tool marks, however a high polish is neither necessary nor desired in our case. Fine wet or dry paper of about 400 grade will do an adequate job. Be careful not to round off any corners which should remain sharp. The barrel should be sanded lengthwise with the paper wrapped around a block of wood to prevent rounding of octagonal ridges.

The formula consists of dissolving about ½ teaspoon Sal Ammoniac, (Ammonium Chloride), and a piece of Copper Sulphate crystal about 1/4” x 1/4” x 1/8” in size in a pint of water.
The parts should be thoroughly degreased by washing with soap and water, followed by a bath of household Oakite or Borax.

COLD RUST METHOD.
Rub the solution on the metal with a clean rag being careful to thoroughly wet all parts to be browned but leave the surface only damp. Puddles or droplets tend to make streaks or spots. Leave the work in a damp place for about 6-8 hours until a coat of rust develops. In dry climate it will help if the work is placed in a box covered with damp rags. The rags must not touch the work. After the surface has rusted over, brush the work off with a fine steel wire brush which has been degreased. Under the loose rust will appear a coat of brown. Repeated applications will darken the finish. The work should be kept at about 75F or higher during the application of the rusting process.

BOILING METHOD.
Bring solution to the boil in an enamelled or Pyrex dish. Dip the work on a wire or string in the solution for 45 seconds. Remove and shake or blow off the excess solution and evaporate dry over the fire, without over heating the metal. Allow work to rust sufficiently to produce a solid coat of rust and then remove rust. One application generally gives a good brown. Repeated applications or leaving in the boiling solution longer tends towards a reddish-purple, then blue and black. This method is very good for locks, screws. Etc.

HEAT METHOD.
Heat the work until it will almost cause water to sizzle-about as hot as a pressing iron for clothes.
Dampen a small pad of rag in the solution and rub on the work, allowing it to evaporate off rapidly. Work small areas and notice that the brown will become apparent after about 10 passes.
Continue over the work until the desired finish is obtained. This method does not require the rusting period, although the finish seems to darken and its resistance to wear is improved by allowing it to age for 24 hours before handling. If it rust some during this period, the rust can be removed as described above. It can be oiled immediately after obtaining the desired colour without affecting the finish, which will reduce after-rust. After obtaining the desired colour with any of the three methods, the work should be washed with soap and water to stop after-rusting.