code: public static function toBlock(Serializer $serializer, $json)
      {
          if (empty($json)) {
              return null;
          }
  
          $contentArray = json_decode($json, true);
          if (!array_key_exists("type", $contentArray)) {
              return null;
          }
  
          $className = BlockFactory::getBlockClass($contentArray["type"]);
          if (!class_exists($className)) {
              return null;
          }
  
          return $serializer->deserialize($json, $className, 'json');
      }
docs: De-serializes a block
  
  @param \JMS\Serializer\Serializer $serializer
  @param $json
  
  @return \RedKiteCms\Content\Block\BaseBlock