Just for fun: Wych elm, with dark, scaly bark and heartwood and interlocking grain ("That braided grain was what gave bows made from it such power, twice what any other wood could give.") Elm is definitely more "braided grain" than yew: Pacific Yew, at least where I live, has dark outer bark, very dusty, until you peel of the outer layer. Growing in dark patches of fir, it could be considered "Black". Or possibly 'black" refers to the poison nature of the yew, and is a colloquialism used by the Two Rivers folk? OR: make up your own reason, like Jordan made up the tree.