14 files declare their use of BlockPluginInterface

BlockComponentRenderArray.php in core/modules/layout_builder/src/EventSubscriber/BlockComponentRenderArray.php
BlockComponentRenderArrayTest.php in core/modules/layout_builder/tests/src/Unit/BlockComponentRenderArrayTest.php
BlockForm.php in core/modules/block/src/BlockForm.php
BlockInterfaceTest.php in core/modules/block/tests/src/Kernel/BlockInterfaceTest.php
BlockSettings.php in core/modules/block/src/Plugin/migrate/process/BlockSettings.php
BlockStorageUnitTest.php in core/modules/block/tests/src/Kernel/BlockStorageUnitTest.php
Broken.php in core/lib/Drupal/Core/Block/Plugin/Block/Broken.php
color.module in core/modules/color/color.module
Allows users to change the color scheme of themes.
ConfigureBlockFormBase.php in core/modules/layout_builder/src/Form/ConfigureBlockFormBase.php
help.module in core/modules/help/help.module
Manages displaying online help.
menu_ui.module in core/modules/menu_ui/menu_ui.module
Allows administrators to customize the site's navigation menus.
SectionComponentTest.php in core/modules/layout_builder/tests/src/Unit/SectionComponentTest.php
SectionRenderTest.php in core/modules/layout_builder/tests/src/Unit/SectionRenderTest.php
system.module in core/modules/system/system.module
Configuration system that lets administrators modify the workings of the site.

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