I have seen Helleborus foetidus, Dicentra, Hosta (particularly 'Royal Standard') and primula do well under black walnuts. I know that the last three are hardy for you, not sure about the helleborus. Oh, and Arum italicum.

We successfully grow iris, day lily, hesperis matronalis (sweet rocket) tall phlox, and lycoris (surprise lily) below a black walnut tree. Also in the area are photonia, flowering quince, forsythia, and mahonia shrubs. However, the tree is tall and the branches do not spread much. good luck. EP

Go to the forum known as the "Plants for Difficult Places". I have a post (Second Report....)reporting the plants I have within the drip line of Black Walnuts. This is a new planting; however, as far as the perennials are concerned, these are on lists of tolerant plant to juglone toxin.
Add to the list, Hydrangea microphelia (?sp) and Kerria japonica. I have these planted beneath a BW and they are doing fine. Also the Hydrangea species variation Annabelle is tolerant. Rugosa roses also seem to be tolerant and will tolerate some shade.

Good luck. It is always a challenge to plant anything with these trees.

I was interested in the replies to the question about suitable plants to grow near or under Black Walnuts, as we moved last fall to a property with a huge Black Walnut near the house. I was surprised to see a lilac flourishing around the trunk, as some sites have listed lilacs as one plant that will not grow under BWs. I don't know how robustly it flowers in the shade, but it seems perfectly healthy, and I was told it's been growing there "forever".
------------------------------------------------------------
I plan to look at the "plants for difficult places" forum also, but wondered if anyone has suggestions for other uses for the area beneath the tree. I had thought of locating a small chicken house nearby, but dont know if the leaves are a problem for poultry (not to mention the walnuts dropping on the roof and scaring them half to death!). Part of the area is a gravelled parking area at present, but BWs are not always car-friendly! Another consideration is ease of gathering the walnuts, a chore that apparently falls to to one of our neighbors, in exchange for a superb poppy/black walnut bundt cake- a tradition that we'd certainly like to continue ;-)

I have been stuggling with this for years and have learned to try many different things...fortunately, I have no way of knowing if a plant's demise is from the dripline of BW or something else - although I do try most things in more than one area. I have had success with helleborus, hydrangea, foxglove, penstemmon and many of the things on the list on this link. (the list is at the end of the article). One thing I have had trouble with is good old laminum....and I can only suspect juglone toxicity.

Here is a link that might be useful: PLANTING NEAR BLACK WALNUTS