Block machine name is in use that is not available I have a Site Branding block I style in my theme. To do this I give it a specific machine name, in this case: sitelogo 

On a site using that theme I am trying to create this block but when I try to edit the machine name of the block to sitelogo I am getting the following error:

The machine-readable name is already in use. It must be unique.


An actual block with that machine name does not appear to exist, or at least I cant find it on the site. 

Is there a way to force the deletion of that machine name so I can re-use it? I am pretty sure I didn't create it and I need block to have that specific machine name...
Block machine names must be unique in the system, not only in the theme.

When you know the machine name of a placed block you can delete it in UI: 

/admin/structure/block/manage/sitelogo/delete


This will of course remove the site logo from one of the other themes.