A blacklist is a list you can be registered on if spam email or malicious applications are registered on your domain.
Most often, a blacklist will register the IP address of the domain (A-record). The IP address is the virtual address of the server itself.
It also means that if you use shared hosting, other people’s domains and applications can directly affect you and your domain, and vice versa.
Blacklists are used today by many email providers, to sort out spam mails, or in some cases to completely block an IP address from delivering emails.