When having an existing space that you want to use as a Space Blueprint there is a workaround to do so.
Create a new Space Blueprint (eg. New Department) and access the Space view. Click on Overview in the Space tools to see the Space Details. Choose Edit Space Details and set the status from Archived to Current.
(Technically, a Space Blueprint is a common space which the Blueprint Creator automatically archives so it isn’t listed in the Space Directory or content from it won’t be indexed when users use the search.)
Access the space you want to use as a Space Blueprint (eg. Department), navigate to the Space Dashboard/homepage and move (or copy) the page (child pages are included automatically) to your Space Blueprint which is visible because of Step 1.
All pages that you initially created in the original space are now available in your new Space Blueprint.
Go back to your new Space Blueprint (eg. New Department) and click on Overview in the Space tools to see the Space details again. Choose Edit Space Details and set the status from Current to Archived.
Your Space Blueprint is now ready to be used. Of course you can customize it further and add placeholders if you like. If you don’t need your original space anymore you can delete or archive it.