Most maple species are resistant to the allelopathic effects of black walnuts, Japanese maples included. I would still have concerns about planting these trees where they would encounter a lot of root competition from the walnuts - the competition will slow establishment and there's a risk of the walnuts hogging all the soil moisture and nutrients. I'd provide additional attention to both watering and mulching to supply nutrients.

Thank you for your information. I appreciate it. Carol

I don't think there will be that much root competition with a walnut. I have over 500 hostas under a canopy of mainly walnut trees and they do fine. I chose the BW due to the fact that they are taprooted trees and don't have very many surface roots.
Do a google search for more information or call your local forestry office.

Whether or not a tree develops a so-called tap root has little bearing on the appearance and location of the rest of the root system. All trees will concentrate the bulk of their roots very close to the soil surface, typically within the top 12-18", with most of the very fine and dense feeder roots directly below the soil surface. That's where the oxygen, soil moisture and nutrients are located and so that's where the roots predominate. Any forestry or arboriculture resource will confirm this.

It is one thing to establish smaller growing, very shallowly rooted perennials like hostas under the canopy of any large tree -- it is quite another to plant another tree or large shrub. The root systems will compete and the larger one usually wins unless special care is provided for the smaller species until it is well-established. Often this need for additional attention will persist, especially with drought intolerant species like Japanese maples.

If you plant them too close, they will lean, or stretch for the sun.

Which side of the walnuts are you fixing to plant the maples on?

My theory is the south side of a row of walnuts will have more sun than the north side. Think about the old moss on the north side of the tree trunk thing.

Black Walnuts in my yard get their leaves late and dump them early.