First, thanks so much, team! This is a wonderful addition to Guide.
The following is a bit clumsy, but not prohibitively so. It might suffice simply to document it better:
Through exploration, I've learned that when an article uses Content Blocks (CBs), then the content between any two adjacent CBs is treated as an independent HTML document. A few implications of this:
1) You can't use the keyboard to navigate through / around / past a CB. For example, if you press down-arrow repeatedly, the cursor stops when it gets to just before a CB.
2) You can't copy/paste CBs to a different location in the article. You either have to copy/paste text around the CBs, or remove the CB and re-insert it elsewhere.
3) The HTML editor only shows you the portion of the article between two adjacent CBs (based on where your cursor was when you opened the HTML editor). So each inter-CB segment must be edited separately.
4) If you insert a CB at the very end of an article, you can't then insert any text after the CB (because you can't get the cursor past the CB to start typing). Of course an easy workaround is to add a blank line first, and insert the CB before that blank line.
Thanks again!