Scarlet elder grows well enough for me, but it is a shrub of cooler climates. It definitely is not as vigorous as common elderberry, requiring cool, moist conditions for best growth. (it is pretty, though!)

You might try Maple Leaf Viburnum - Around here, it is one of the best shrubs for shade. It seems to take black walnut all right.

Another interesting shrub is wild hydrangea (Hydrangea arborescens) - Especially the cultivars like "annabelle" and "grandiflora". They do need constant moisture, though.

I like the viburnum family as a whole. Maybe I'll order another mix and see what takes up there. Thanks for the tip.