A few years ago,I came across and old Marlin 94, 38-40 made in about 1906. It looked about the same. I used a cold blue on it from Brownell's ( Oxpho Blue)with particular attention to detail as to the process.
Heat the metal and degrease.
Apply solution generously and let set overnight.
Buff with a cotton rag for a more blue color or steel wool for a deeper black.
Repeat process if it is streaked and not deep enough.
It isn't a very robust blueing,but will give the metal some color and you can retouch anytime.