{% block content %} is a no-go in template/subfolders
In this case, it was just human error and ignorance of how hidden template files work through the CMS. 

I believed that I should be able to call the hidden file in the browser, via something like "base_url/subfolder/_hiddenfile.html" and partially I was able. For whatever reason, using this primitive test method, I was able to see the header and footer, but not the block content, in the browser window. 

The fact is, what I was doing was not a viable test. I don't understand why, but that doesn't matter.