Blackholing is an anti-spam technique in which an Internet service provider (ISP) blocks packets coming from a certain domain or address. Blackholing can also refer to an individual who sets up a similar barrier up for his or her personal network. Blackholing of specific domains can prevent certain types of malware and denial of service attacks.
Blackholing is essentially a security measure that can be put into place against known offenders. By blackholing certain IP addresses and domains, an ISP can reduce the amount of spam entering the system, reducing the load and making customers happy in the bargain. Blackholing does, of course, have the potential to be abused by ISPs, which could use it to block competing services or prevent legitimate activities.