I didn't know that these caches won't be published. Do you know which part of the guide lines mentions that?
What I mean blackout is that you have to find all the caches in Duvall with a few exceptions.
I only see 12 caches within the city of Duvall. That would not be much of a challenge. If you wanted to add other caches that in the area, you would have to make a specific list of caches. That is not allowed. You are allowed to simply say, find all published caches in Duvall. A dedicated cacher could knock that out in less than two hours.
Actually, there are some around Duvall, but they are still in Duvall city limits so I counted these. There is also a boat cache, so it would take longer to complete that challenge if I included it.