I was in Blick recently when I spotted a bottle labeled 'Walnut Ink'. Intrigued, I had to pick it up. As soon as I came home I had to start messing around with it! I found that it was not very 'saturated', which gave it a nice variation in depth of color. It lends itself more to line/brushwork than larger fields, as it dries slowly and the pigment settles unevenly.
Walnut Ink is made by boiling the green outer husk of a walnuts and then reducing that liquid by boiling it under low heat for several hours. There's a recipe here http://www.fountainpennetwork.com/forum/index.php/topic/232797-homemade-black-walnut-ink/ (mostly posting that cause I love the lady's supercreepy profile picture)