Blocklist means essentially the same thing as "blacklist". As the term is used in the email marketing industry, it means a set of items that are rejected. This can be specific email addresses, or all email addresses from a specified domain name, or from a specified IP address, and so on. It can be devastating to one's email marketing efforts to be put on a "blocklist." When that happens, one's email is no longer accepted by various mail servers (gmail, hotmail, AOL, comcast, etc.) This is one reason why using services like Mailchimp or Constant Contact can be very valuable; their policies of use take care not to violate the CAN-SPAM act or good manners and so your own mail server does not get put on blocklists and your emails do get through. It can be quite frustrating to get OFF a blocklist once one has been put on such a list.