7 files declare their use of BlockInterface in Drupal 9

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.