8 files declare their use of BlockContentInterface

BlockContent.php in core/modules/block_content/src/Entity/BlockContent.php
BlockContentCreationTest.php in core/modules/block_content/tests/src/Functional/BlockContentCreationTest.php
BlockContentEntityChangedConstraintValidator.php in core/modules/block_content/src/Plugin/Validation/Constraint/BlockContentEntityChangedConstraintValidator.php
BlockContentGetDependencyEvent.php in core/modules/block_content/src/Event/BlockContentGetDependencyEvent.php
block_content.module in core/modules/block_content/block_content.module
Allows the creation of content blocks through the user interface.
ContentImportTest.php in core/tests/Drupal/FunctionalTests/DefaultContent/ContentImportTest.php
MigrateCustomBlockTest.php in core/modules/block_content/tests/src/Kernel/Migrate/d7/MigrateCustomBlockTest.php
SetInlineBlockDependency.php in core/modules/layout_builder/src/EventSubscriber/SetInlineBlockDependency.php

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