I have a couple of cameras / film backs with mirror-nickel or chromed pressure plates: one is a "Kodachrome back" for 9x12cm European plate cameras
( the roll-film part is the body from a Kodak 35 ! ), I believe the other is a Graphic 35 RF camera from the 1950's...
As Mr. Flibble mentioned, so long as your films have decent anti-halation layers, the brassing of the Leica pressure plate should not be a big issue... ( at least not as big an issue as lens-flare from period Leica lenses... 😀 ).
If you do elect to chemically darken the pressure plate, it would probably be best to remove it from the camera, strip the existing blackening, then re-blacken the whole surface, to achieve an even result.
I'm sure any greases or skin oils will cause uneven blackening, so follow the instructions very carefully, and don't chimp on the preparation.
Let us know how things turn-out...
LF