Seven subtheme not inheriting correct block structure I am using Drupal 8.

I have subthemed the Seven theme so we can add some minor customisation to the admin theme. 

The new theme however is getting all the active theme's blocks thrown into - seemingly - random blocks inside its regions.

I want to create a subtheme which initially is identical to seven and can be activated across several sites with predictable block placement. 

I don't want users to be put in a position were they have to go through and manually move/disable blocks after turning on the new admin theme. This is especially true given the clunky nature of adjusting block placement on the admin theme; every time you disable/delete a block and save the change it takes you back to the ACTIVE theme block page rather than the admin one...

Is there a way to do this?
You can provide custom block configurations in config/install as with a module. As soon as there is at least one block, Drupal will not add any additional blocks, if there are none, it will copy those from the default theme.