Issue #2248951 by eojthebrave: Missing documentation for block plugin annotations.

core/modules/block/lib/Drupal/block/Annotation/Block.php

@@ -32,4 +32,20 @@ class Block extends Plugin {
    */
   public $admin_label = '';
 
+  /**
+   * The category in the admin UI where the block will be listed.
+   *
+   * @var \Drupal\Core\Annotation\Translation
+   *
+   * @ingroup plugin_translatable
+   */
+  public $category = '';
+
+  /**
+   * Class used to retrieve derivative definitions of the block.
+   *
+   * @var string
+   */
+  public $derivative = '';
+
 }

core/modules/block/lib/Drupal/block/Plugin/Type/BlockManager.php

@@ -61,7 +61,7 @@ class BlockManager extends DefaultPluginManager {
     parent::processDefinition($definition, $plugin_id);
 
     // Ensure that every block has a category.
-    if (!isset($definition['category'])) {
+    if (empty($definition['category'])) {
       $definition['category'] = $this->getModuleName($definition['provider']);
     }
   }