Where to plug in a new block in default blog (or other) page

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(Version 5.6.1.2) I am trying to add AddThis to my site and have become flummoxed over how to modify the basic page layouts. I used this sequence in the dashboard:

Pages & Themes
Page Types
Blog Entry> Defaults

This allowed me to add an HTML block to the sidebar, into which I pasted the AddThis code. The expected "Share" button appeared in the sidebar and I saved it. If I return to this default, the button is still there, but there has been no change to my existing blog pages. What have I missed? I tried adding the same block to my site's main page and it worked fine, so the block itself seems okay.

On a related note, the other three page types don't allow any modifications at all. Although I clearly need a refresher course in how inheritance works in this CMS, I could use a pointer or keyword for finding it in the support documentation. Thanks!

 
jcarls replied on at Permalink Reply
Never mind, I found the answer to the main question. You have to edit the page, click on the specific block and select "Setup on child pages" to propagate the change. For the record then, the note that appears at the top of the page while editing ("Page Defaults for Blog Entry Page Type. All edits take effect immediately.") really needs some clarification.

This does not answer the second half of my question, which is: Why do the "left sidebar" and "right sidebar" page types not allow any changes at all, even though you can invoke "editing" mode for them?