Pictures may help; is it possible the forend is just darkened from being oil soaked? many older guns are that way. I have personally used acetone and white spirit to remove oil usually just sit the forend in a pint glass full of it for a week.If that does lighten enough i have heard of people use bleach to lighten wood, to good effect.
Surely it is easier to lighten the for end than risk ruining or stripping the finish on the butt stock. At least then if it goes wrong you only have to re finish a for end and not the entire but stock.
If you are still considering darkening the butt stock. I use alkanet root in my stains, but usually this is on raw un oiled timber. First coats are a mixture of alkanet root, acetone, and linseed oil, which makes a thin mixture. After a few coats of this i go to straight BLO with alkanet. After a few coats of this i use my standard BLO mixture. In my experience this will not penetrate through an existing finish and needs to be applied to stripped wood.
I have never heard of using alcohol based stains over an oiled finish, i would have thought this would strip the finish rather than stain the wood.