code: public function checkBlacklistedRecipient($varValue, Contao\DataContainer $dc)
  	{
  		$objBlacklist = $this->Database->prepare("SELECT COUNT(*) AS count FROM tl_newsletter_blacklist WHERE hash=? AND pid=(SELECT pid FROM tl_newsletter_recipients WHERE id=?) AND id!=?")
  									   ->execute(md5($varValue), $dc->id, $dc->id);
  
  		if ($objBlacklist->count > 0)
  		{
  			throw new Exception($GLOBALS['TL_LANG']['ERR']['blacklisted']);
  		}
  
  		return $varValue;
  	}
docs: Check if a recipient is blacklisted for a channel
  
  @param mixed                $varValue
  @param Contao\DataContainer $dc
  
  @return mixed
  
  @throws Exception