Is there a theme equivalent of hook_install()?
You may provide configuration via YAML files as part of a module, profile or theme install by adding them to the config/install directory inside the respective project. So, for example, a file block.block.my_theme_branding.yml would install the branding block into the specified region. This would then contain the setting to disable the site slogan by default. Take a look at block.block.bartik_branding.yml in the standard profile as a starting point.

If the specific block is provided by another module, then it should probably be placed in the config/optional directory.