What is an email Blacklist?
An email Blacklist is a real-time database that uses criteria to determine if an IP is sending email it considers to be SPAM.
Each list will have it's unique way of accepting inbound mail and determining if email is considered SPAM. They can all impact email deliverability.
How do Blacklists work?
This diagram from Spamhaus will show the process for how a blacklist works. If inbound mail does not meet requirements to be determined legitimate mail, then it is added to their blacklist.
Some ISPs will use this data to influence their filtering decisions and delivery for email sent from that IP. The blacklist is a tool that helps ISPs make filtering decisions, they do not control the delivery or placement of your emails.