Typically, juglone-sensitive landscape plants should be located at least 60 feet from black walnut trees to avoid damage. Raised beds near trees will help minimize toxicity, but care should be taken to keep leaves, twigs, branches, and nuts out of the bed. Black walnut leaves, bark, and wood chips should not be used as landscape mulch or composted on garden plants. I'm a little surprised that 75 feet away from the walnut tree isn't far enough, especially since hydrangeas are considered to be only moderately susceptible.

Juglone acts as a respiration inhibitor. Affected plants cannot exchange carbon dioxide and oxygen properly, which deprives them of needed energy for metabolic activity. The resulting wilting cannot be reversed.

Where walnuts crowd too close, build raised beds with root barriers in the bottom — concrete or rubber patio blocks are one option. Landscape fabric liner may not be sufficient. Hefty tubs
are another option. You could raise those beds even higher: built at waist height, they could keep your back happy too--and would provide plenty of room for your hydrangea roots.

The best alternative is to choose plants from lists of plants thought to be tolerant of juglone. The information in the links below lists shrubs (and other plants) that are tolerant of walnut juglone.

https://www.johnson.k-state.edu/docs/lawn-and-garden/in-house-publications/trees-shrubs/Landscaping%20Near%20Black%20Walnut%2

https://hortnews.extension.iastate.edu/faq/what-plants-are-sensitive-juglone-produced-black-walnuts

I guess I should have asked you if you are sure the problem with your hydrangeas is the walnut toxicity?

Most toxicity symptoms arise when juglone-sensitive plants are placed within the walnut’s root zone, an average of 50 to 60 feet from the trunk of a large tree. Plants sensitive to juglone may show signs of decline (wilting, yellow leaves, or stunted growth). Many highly sensitive plants can react negatively to even a small concentration of juglone. There are other factors that may impact the health of plants and those should be considered before black walnut toxicity is assumed to be the problem.

Consider such things as soil pH, sun/shade location, etc. You may want to conduct a soil test to assure all the cultural requirements for hydrangeas are present. Self-mailer soil test kits from the Michigan State University Plant and Pest Diagnostic Lab are available. You can obtain them at the link below. Full instructions are there and also come with the kits. The cost is $25. The Lab will make recommendations for any needed soil amendments. Be sure to ask them if they can test for walnut juglone.

https://shop.msu.edu/product_p/bulletin-e3154.htm

I am 99% it is Juglone toxicity.  Have no tests to make 100% certainty.   I have 80 others on othersides of building with no similar  problems.   Tree has an 8 to 10 foot diameter so I am  guessing close to 200 yrs old or older. My house  is at least  118yrs old. For 60 years there has been a privacy fence which trapped leaf debris before I  cleaned it out 3 yrs ago and tried growing Paniculatas  there since I  tried Macrophyllas in other locations years ago and they died.  Upon trying to determined why such healthy  plants rapidly died in such great looking soils when given appropriate  care and why tomatoes would not ripen and friend who is a 2nd or 3rd generation fruit growers told me he bet a black walnut tree was somewheres nearby 

My final question  is:  Do you think carefully washing all dirt off the roots would cause stress to my plants, before I  replant in containers 

Thanks so much for your quick and revelant response 

It would be a lot safer to wash the soil off and replant your hydrangeas if they were dormant, but they may not live that long.

I think it’s worth the risk if you wash off the soil and replant the bare roots immediately in another location. It’s for sure leaving them in the juglone will kill them.

So yes, your plant will no doubt suffer some stress if transplanting but should be ok if you keep them well watered and in good soil for the next few months. And maybe a little sun protection for awhile since we’re just getting into the heat of summer. 

Good luck! It’s nice to converse with a gardener who’s done his research!