From an administrator account, I added testgroup to the permissions for a page. I went to a block in an area and removed testgroup from the permissions. I went back to the block permissions, and it has this message at the top.

"Permissions for this block currently override those of the parent area. To revert to the area's permissions, click the button below."

However, there is no "button below" to revert block permissions to the area permissions (like the button in area permissions to revert to page permissions). Saving the page and editing it, again, results in the same behavior (no button).