7 files declare their use of BlockInterface

Block.php in core/modules/block/src/Entity/Block.php
BlockController.php in core/modules/block/src/Controller/BlockController.php
BlockEntitySettingTrayForm.php in core/modules/settings_tray/src/Block/BlockEntitySettingTrayForm.php
BlockHasOverridesAccessCheck.php in core/modules/settings_tray/src/Access/BlockHasOverridesAccessCheck.php
BlockPluginHasSettingsTrayFormAccessCheck.php in core/modules/settings_tray/src/Access/BlockPluginHasSettingsTrayFormAccessCheck.php
BlockPluginHasSettingsTrayFormAccessCheckTest.php in core/modules/settings_tray/tests/src/Unit/Access/BlockPluginHasSettingsTrayFormAccessCheckTest.php
settings_tray.module in core/modules/settings_tray/settings_tray.module
Allows configuring blocks and other configuration from the site front-end.

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