def goBack(self):
        """
        Goes up one level if possible and returns the url at the current level.
        If it cannot go up, then a blank string will be returned.
        
        :return     <str>
        """
        if not self.canGoBack():
            return ''
        
        self._blockStack = True
        self._index -= 1
        self.emitCurrentChanged()
        self._blockStack = False
        return self.currentUrl()