A blacklist blocks email considered spam, such as unsolicited mails sent by an automated system, from entering your inbox. An email blacklist works by listing IP addresses that spam comes from on a virtual list. Emails sent from blacklisted IP addresses go directly into the spam folder.
ISP's (Internet Service Providers) are primarily responsible for operating the blacklists. If you regularly send emails to people that our not interested in your product or service, you will likely be deemed a spammer and have a strong chance of being blacklisted by ISPs.