I use artists lampblack (which is what they make India ink with) for giving walnut an aged look. Dilute the lampblack with a little water, then rub it in well, once dry, final sand smooth (the black is left in the normal pores which I used to wet sand to fill, ugh, mud duck ugly) and apply finish. It works and looks great. Did not come up with this on my own, read about it on another ML site.
Unless mistaken, Mike Brooks is the originator of using flat black spray paint on walnut stocks, then sanding, for giving a great aged look to them. I will be giving that a try on my next piece of walnut.