there are basically two methods for making walnut dye or ink.
the first is indeed more dye like and may be more useful for yr purpose. certainly less time consuming.
http://www.practicalprimitive.com/skillofthemonth/blackwalnutdye.htmli usually make walnut ink. more time consuming.
the resulting product can be used for many things.
i typically make about a quart each year and it lasts me all year and i share with friends.
http://denevell_books.home.insightbb.com/making_walnut_ink.htmthis is the most like how i do it i could find.
i crush however many hulls i have(usually 20+ walnuts)
two or three quarts of water and simmer overnite or until liquid has reduced to half.
then i strain out the hulls thru and old tshirt.
simmer till liquid is once more reduced by half.
a dash of alum and it wont rot on you.
great stuff to have around and really simple, from readily available materials.