I'm not sure whether I entirely understood your question, but here are the steps I took to create and edit a custom block to appear in the side bar of my test journal. Please let me know whether following these instructions does not help you.
Journal Manager > System Plugins > Generic Plugins > Custom Block Manager (enabled) > Settings > Add Block > enter the title of the block in the text field > SAVE > back to System Plugins > Block Plugins. You should find your new block plugin title in the list. When you select Edit under the title of your new block plugin, you should be able to add your custom text in the free-form text field.
Once you have done that, you may need to check in Step 5.6 of The Look layout section of the Setup in the Journal Management pages to make sure that the new custom block appears in the left or right side bar. If it is in the middle column, it will not appear, that column lists the blocks you choose not to display. Use the arrows to move the custom block to the left or ride side panel to have it appear on either the left or right panel.
Again, please feel free to respond if this process does not work for you.
Electronic Publications Manager
Office of Scholarly Communication and Publishing
University Library System, University of Pittsburghvgabler@pitt.edu