31 files declare their use of Block in Drupal 9

AreaEntityUITest.php in core/modules/views_ui/tests/src/Functional/AreaEntityUITest.php

AssertBlockAppearsTrait.php in core/modules/block/tests/src/Functional/AssertBlockAppearsTrait.php

block.module in core/modules/block/block.module

Controls the visual building blocks a page is constructed with.

BlockConfigSchemaTest.php in core/modules/block/tests/src/Kernel/BlockConfigSchemaTest.php

BlockCreationTrait.php in core/modules/block/tests/src/Traits/BlockCreationTrait.php

BlockDependenciesTest.php in core/modules/views/tests/src/Kernel/Plugin/BlockDependenciesTest.php

BlockFormTest.php in core/modules/block/tests/src/Unit/BlockFormTest.php

BlockInvalidRegionTest.php in core/modules/block/tests/src/Functional/BlockInvalidRegionTest.php

BlockLanguageTest.php in core/modules/block/tests/src/Functional/BlockLanguageTest.php

BlockRebuildTest.php in core/modules/block/tests/src/Kernel/BlockRebuildTest.php

BlockResourceTestBase.php in core/modules/block/tests/src/Functional/Rest/BlockResourceTestBase.php

BlockStorageUnitTest.php in core/modules/block/tests/src/Kernel/BlockStorageUnitTest.php

BlockTemplateSuggestionsTest.php in core/modules/block/tests/src/Kernel/BlockTemplateSuggestionsTest.php

BlockTest.php in core/modules/jsonapi/tests/src/Functional/BlockTest.php

BlockTest.php in core/modules/block/tests/src/Functional/BlockTest.php

BlockViewBuilder.php in core/modules/block/src/BlockViewBuilder.php

BlockViewBuilderTest.php in core/modules/block/tests/src/Kernel/BlockViewBuilderTest.php

BreadcrumbTest.php in core/modules/system/tests/src/Functional/Menu/BreadcrumbTest.php

EntityCrudHookTest.php in core/tests/Drupal/KernelTests/Core/Entity/EntityCrudHookTest.php

InstallerThemesBlocksProfileTest.php in core/tests/Drupal/FunctionalTests/Installer/InstallerThemesBlocksProfileTest.php

LanguageUILanguageNegotiationTest.php in core/modules/language/tests/src/Functional/LanguageUILanguageNegotiationTest.php

LocaleConfigManagerTest.php in core/modules/locale/tests/src/Kernel/LocaleConfigManagerTest.php

MenuUiTest.php in core/modules/menu_ui/tests/src/Functional/MenuUiTest.php

MigrateBlockTest.php in core/modules/block/tests/src/Kernel/Migrate/d6/MigrateBlockTest.php

MigrateBlockTest.php in core/modules/block/tests/src/Kernel/Migrate/d7/MigrateBlockTest.php

NodeBlockFunctionalTest.php in core/modules/node/tests/src/Functional/NodeBlockFunctionalTest.php

OverriddenConfigurationTest.php in core/modules/settings_tray/tests/src/FunctionalJavascript/OverriddenConfigurationTest.php

SettingsTrayTest.php in core/modules/settings_tray/tests/src/Functional/SettingsTrayTest.php

SettingsTrayTestBase.php in core/modules/settings_tray/tests/src/FunctionalJavascript/SettingsTrayTestBase.php

SystemMenuBlockTest.php in core/modules/system/tests/src/Kernel/Block/SystemMenuBlockTest.php

WhosOnlineBlockTest.php in core/modules/user/tests/src/Kernel/WhosOnlineBlockTest.php