7 files declare their use of Block

AdminBlockDeleteForm.php in drupal/core/modules/block/lib/Drupal/block/Form/AdminBlockDeleteForm.php
Contains \Drupal\block\Form\AdminBlockDeleteForm.
block.admin.inc in drupal/core/modules/block/block.admin.inc
Admin page callbacks for the block module.
BlockListController.php in drupal/core/modules/block/lib/Drupal/block/BlockListController.php
Contains \Drupal\block\BlockListController.
BlockPluginBag.php in drupal/core/modules/block/lib/Drupal/block/BlockPluginBag.php
Contains \Drupal\block\BlockPluginBag.
BlockStorageUnitTest.php in drupal/core/modules/block/lib/Drupal/block/Tests/BlockStorageUnitTest.php
Contains \Drupal\block\Tests\BlockStorageUnitTest.
overlay.module in drupal/core/modules/overlay/overlay.module
Displays the Drupal administration interface in an overlay.
ViewTestBase.php in drupal/core/modules/views/lib/Drupal/views/Tests/ViewTestBase.php
Contains \Drupal\views\Tests\ViewTestBase.