Follow up to #1927608: Remove the tight coupling between Block Plugins and Block Entities. Even prior to that patch, blocks had a 'settings' key in their CMI files, but that patch expanded that key to encompass all configuration that needs to be passed to the plugin. However, the plugin API refers to configuration that's passed to plugins as $configuration. In that issue, that variable name also ends up getting passed to theme('block'). In #1927608-197: Remove the tight coupling between Block Plugins and Block Entities, catch identified that the mismatch in variable names ('settings' within the CMI files, but 'configuration' within hook_preprocess_block()) is a DX problem.
Discuss.