diff -u b/core/modules/block_content/src/Entity/BlockContent.php b/core/modules/block_content/src/Entity/BlockContent.php
--- b/core/modules/block_content/src/Entity/BlockContent.php
+++ b/core/modules/block_content/src/Entity/BlockContent.php
@@ -177,8 +177,7 @@
       ->setDisplayOptions('form', array(
         'type' => 'language_select',
         'weight' => 2,
-      ))
-      ->addConstraint('UniqueField');
+      ));
 
     $fields['info'] = BaseFieldDefinition::create('string')
       ->setLabel(t('Block description'))
@@ -191,7 +190,7 @@
         'weight' => -5,
       ))
       ->setDisplayConfigurable('form', TRUE)
-      ->addConstraint('BlockContentInfo', []);
+      ->addConstraint('UniqueField', []);
 
 
     $fields['type'] = BaseFieldDefinition::create('entity_reference')
only in patch2:
unchanged:
--- /dev/null
+++ b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldConstraint.php
@@ -0,0 +1,31 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldConstraint.
+ */
+
+namespace Drupal\Core\Validation\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+
+/**
+ * Checks if an entity field has a unique value.
+ *
+ * @Constraint(
+ *   id = "UniqueField",
+ *   label = @Translation("Unique field constraint", context = "Validation"),
+ * )
+ */
+class UniqueFieldConstraint extends Constraint {
+
+  public $message = 'A @entity_type with @field_name %value already exists.';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validatedBy() {
+    return '\Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldValueValidator';
+  }
+
+}
only in patch2:
unchanged:
--- a/core/modules/block_content/src/Plugin/Validation/Constraint/BlockContentInfoConstraint.php
+++ /dev/null
@@ -1,31 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\block_content\Plugin\Validation\Constraint\BlockContentInfoConstraint.
- */
-
-namespace Drupal\block_content\Plugin\Validation\Constraint;
-
-use Symfony\Component\Validator\Constraint;
-
-/**
- * Supports validating custom block names.
- *
- * @Constraint(
- *   id = "BlockContentInfo",
- *   label = @Translation("Custom block name", context = "Validation")
- * )
- */
-class BlockContentInfoConstraint extends Constraint {
-
-  public $message = 'A block with description %value already exists.';
-
-  /**
-   * {@inheritdoc}
-   */
-  public function validatedBy() {
-    return '\Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldValueValidator';
-  }
-
-}
only in patch2:
unchanged:
--- a/core/modules/block_content/src/Tests/BlockContentValidationTest.php
+++ b/core/modules/block_content/src/Tests/BlockContentValidationTest.php
@@ -37,7 +37,7 @@ public function testValidation() {
     // Make sure the violation is on the info property
     $this->assertEqual($violations[0]->getPropertyPath(), 'info');
     // Make sure the message is correct.
-    $this->assertEqual($violations[0]->getMessage(), format_string('A block with description %value already exists.', [
+    $this->assertEqual($violations[0]->getMessage(), format_string('A custom block with block description %value already exists.', [
       '%value' => $block->label(),
     ]));
   }