diff -u b/core/modules/block/lib/Drupal/block/BlockBase.php b/core/modules/block/lib/Drupal/block/BlockBase.php
--- b/core/modules/block/lib/Drupal/block/BlockBase.php
+++ b/core/modules/block/lib/Drupal/block/BlockBase.php
@@ -10,6 +10,7 @@
 use Drupal\Core\Plugin\PluginBase;
 use Drupal\block\BlockInterface;
 use Drupal\Component\Utility\Unicode;
+use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Language\Language;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Cache\CacheableInterface;
@@ -44,9 +45,11 @@
    * {@inheritdoc}
    */
   public function setConfiguration(array $configuration) {
-    $this->configuration = $configuration +
-      $this->defaultConfiguration() +
-      $this->baseConfigurationDefaults();
+    $this->configuration = NestedArray::mergeDeep(
+      $this->baseConfigurationDefaults(),
+      $this->defaultConfiguration(),
+      $configuration
+    );
   }
 
   /**