I'd try a solution of oxalic acid on a Q-tip to see if that will remove it. But I'd probably first try staining a piece of scrap walnut with some cold bluing to have something to experiment with. before taking any chances with my gunstock. I have no actual experience with this problem, so no guarantees it would work.
Oxalic acid will probably lighten or bleach the wood somewhat, so even if it removed the bluing color, it might be necessary to wipe a bit of stain on the area that was cleaned before refinishing. Neutralize the oxalic acid with a basic pH baking soda or ammonia solution, rinse, and dry.
