I don't know about black walnuts as we don't really have them in France, but I would assume it is the same as the ordinary walnut Juglans Regia which happens to be abundant in the county where I currently live - in fact, around here it is a cash crop.
The French folks have been using walnut for staining, dyeing, painting, writing etc for hundreds of years. Here, it is called " brou de noix".
Anyway, I can tell you what my grand mother and her mother before her used to do if it is of any interest. Here goes:
It is the green outer shell of the walnut that is used. You can use the green immature walnut in the spring or the green more mature outer shell that falls off in the autumn to reveal the walnut. Either one will work fine but be aware that anything that comes into contact with it will be stained, including your skin as mentioned in the post above. So use gloves, old containers and spoons, wear old clothes or an apron especially when brewing the concoction..
Whether you use the spring or fall shells, the method remains the same, although my preference is for the autumn one so that you do not waste the fruit.
Collect as many green shells as you want/need, put in a pan and cover with water. Bring to the boil then simmer gently for 2 hours. Strain the resulting liquid twice through a cheese cloth, old t-shirt or paper coffee filter. Put in a jar/bottle with a couple of drops of clove essential oil to prevent mould and/or bacteria and keep or use as needed.
For wood staining, use a fine paint brush to apply lightly. There is no need to soak your furniture with it and no warping will take place. Add a second coat when dry, if you wish for a darker shade. you can then use any preferred method for preserving, i.e oil, varnish, wax, etc. In the olden days, wax was the method of choice.
You can dye your hair. I would not recommend rinsing your hair with it, a) because your scalp will get stained as well as any skin that the concoction will touch and b) because you may have an allergic reaction to it.
My grand mother used to dip a comb into the brew and brush it through her hair, repeating every day until she liked what she saw. I would try on a small section of hair just to see if you are happy with the result before embarking on a whole head job.
Of course you can dye cloth with it but unless you need or are very fond of brown I would not personally bother.
Also, at a time when they still used nibs, it made a fine ink. I happened to have seen some fine monochrome water painting made with that brew as well!
Now, of course, being French and loving the good life, we tend to use those shells to make a fine liqueur!
