6 files declare their use of BlockPluginInterface in Zircon Profile 8.0

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

Contains \Drupal\Tests\block\Unit\BlockRepositoryTest.

block_test.module in core/modules/block/tests/modules/block_test/block_test.module

Provide test blocks.

color.module in core/modules/color/color.module

Allows users to change the color scheme of themes.

help.module in core/modules/help/help.module

Manages displaying online help.

menu_ui.module in core/modules/menu_ui/menu_ui.module

Allows administrators to customize the site's navigation menus.

system.module in core/modules/system/system.module

Configuration system that lets administrators modify the workings of the site.