Hello,
The only relationship between blog and blog post documents is that the blog document is one of the parent documents of the blog post document. If there is a need for the blog document, Kentico CMS goes through all parent documents and tests if the current document is blog. I would recommend that you execute the following method:
CMS.Blogs.BlogHelper.GetParentBlog(int documentId, bool selectOnlyPublished) and enable the SQL debug to see what query gets executed by this method:
SQL queries debugging.
Best regards,
Michal Legen