diff --git a/core/modules/block/block.install b/core/modules/block/block.install
index a9698e5..f473c23 100644
--- a/core/modules/block/block.install
+++ b/core/modules/block/block.install
@@ -12,6 +12,14 @@
 
 /**
  * Update block visibility context mapping.
+ *
+ * This update function updates blocks for the change of
+ * https://www.drupal.org/node/2354889
+ *
+ * Core visibility context plugins are updated automatically; blocks with
+ * unknown plugins are disabled and their previous visibility settings are saved
+ * in key value storage; see change record https://www.drupal.org/node/2527840
+ * for more explanation.
  */
 function block_update_8001() {
   // These 3 are all the contexts that Drupal core provides.
@@ -63,6 +71,8 @@ function block_update_8001() {
             unset($visibility[$condition_plugin_id]);
             continue;
           }
+          // We replace the previous format of "{$context_id}"
+          // with "@{$service_id}:{$unqualified_context_id}".
           $new_context_id = explode('.', $context, 2);
           $condition['context_mapping'][$key] = '@' . $context_service_id_map[$key] . ':' . $new_context_id[1];
         }
diff --git a/core/modules/block/src/Tests/Update/BlockContextMappingUpdateTest.php b/core/modules/block/src/Tests/Update/BlockContextMappingUpdateTest.php
index 772dfb1..c3de576 100644
--- a/core/modules/block/src/Tests/Update/BlockContextMappingUpdateTest.php
+++ b/core/modules/block/src/Tests/Update/BlockContextMappingUpdateTest.php
@@ -80,7 +80,7 @@ public function testUpdateHookN() {
     $disabled_block = Block::load('thirdtestfor2354889');
     $this->assertFalse($disabled_block->status(), 'Block with invalid context is disabled');
 
-    $this->assertEqual(['thirdtestfor2354889' => ['missing_context_ids' => 'baloney.spam', 'status' => TRUE]], \Drupal::keyValue('update_backup')->get('block_update_8001'));
+    $this->assertEqual(['thirdtestfor2354889' => ['missing_context_ids' => ['baloney.spam'], 'status' => TRUE]], \Drupal::keyValue('update_backup')->get('block_update_8001'));
 
     $disabled_block_visibility = $disabled_block->get('visibility');
     $this->assertTrue(!isset($disabled_block_visibility['node_type']), 'The problematic visibility condition has been removed.');