I recently removed a medium-sized black walnut tree from my very small back
yard, mainly because it blocked the only avenue for available light that I
still had any control over, but only after much thought and research. Here's
a link that was helpful to me and convinced the historic commission to let me
do it! It lists a lot of plants that have been observed growing under black
walnuts. I'm sure there will be some exceptions to this list, though.