A blacklist is a list of domain / ip / mac address that are not trusted and should be blocked from any activities.
It happens when:
They are not on trusted list.
When an IP is blacklisted because of a bad reputation (if you are a spammer or if you are a email provider), you need to change it.
They are used to classify or not traffic as good or bad.
For email, they are used to classify a message as spam or not. They are therefore found in Anti-Spam products.
Blacklist database systems return an opinion if an inbound email should be accepted (based on IP Address, …)
A DNSBL is a Domain Name System (DNS) Black List (BL) ie:
It's also known as:
Level 1 denylist at http://wget-mirrors.uceprotect.net/rbldnsd-all/dnsbl-1.uceprotect.net.gz
Spamhaus has several DSN blacklist zone:
The Zen for IP listing that has 3 sub-zones are:
The Domain Block List (DBL) zone: