Birchwood Casey Brass Black will tarnish it as dark as you want it. Best to clean it real goo first. Do not get it on fresh wood or it will turn the stock black or blue & it won't come off. I have used it several times & I finish the rifme, then carefully steel wool the brass & apply with Q'tip, then wipe off & wrok lightly with burlap or 0000 wool. (Must be carefull with the wool as you will easily end up right back where ya started.
Another way is to clean the brass well, wipe with some patches that have fouling on them that you have used to swab with using just spit to swab & load, so there is not grease or oil in it. In a week it will have a nice patina & continue as long as you don't let oiled fingers touch the brass.
Birdseye Maple is good gunstock wood & also the hardest most contrary wood I have ever worked with. It is Definately not a wood for the first time builder.. Extremely hard to keep the surface even as the birdseyes are very hard & in between softer, thus the whole dang stock will washboard Very easily. Also extremely hard to carve & shape, again because of the birdseyes. You have to cut both ways on every dang eye as it will not let your tool cut smooth straight thru......
Basically, it is a real PITA to work with & if you carve, you will most likely ever only do one of these.... :imo: