The term blacklisting is generally used in a negative context, since it implies that someone has been prevented from having legitimate access to something due to the whims or judgments of another. For instance, if a person is served with a restraining order for having threatened someone else, this is generally not considered to be a form of blacklisting, but if someone is fired for exposing poor working conditions in a particular company, and is blocked from finding work in that industry, that is often considered a form of blacklisting.
One of the most famous examples of blacklisting in American history stemmed from the HUAC's investigations into known Communists, among which were the "Hollywood Ten", a group of screenwriters who were believed to be Communists. Among them was Dalton Trumbo, who was barred, or blacklisted, from working in Hollywood due to these associations. (He later worked under a pseudonym and wrote the script for Spartacus, among other films.)
Another word for blacklisting is blackballing. This term allegedly derives from the practice of some private clubs of allowing any member to veto the application for membership of any person by anonymously placing a black ball among the white balls put in an urn by other members when the individual's application was brought up for consideration.
In computer terminology a blacklist is an access control mechanism, that stands everybody, except members of the blacklist. The opposite whitelist means nobody, except members of the white list.
If an organization keeps a black list of software, titles on the list will be banned, everything else will be allowed. A school might black list KaZaA and ICQ, other Internet services are allowed.
If a spam filter may keep a black list, mail from the listed addresses will be filtered away. "Black list" as a verb means put on a black list.