The book functionality allows you to specify even/odd page starts for chapters, and allows you to insert a blank page if a chapter doesn't fall on the desired type of page. But nowhere can you select the master page to be used for the inserted page.

This renders the feature crippled for a great many documents. For example, manuals and legal documents typically require "This page intentionally blank" on blank pages, to assure the reader that the document is complete. This is standard: https://en.wikipedia.org/wiki/Intentionally_blank_page

The question has been raised of what happens if the parent you specified for blanks doesn't exist in the file that needs a blank page appended.

The most straightforward solution (and easiest to implement) would be to add the "blank"-page parent selector to the indd file instead of to the book. Suggested UI attached.