A blacklist is a list or register of entities who, for one reason or another, are being denied a particular privilege, service, or mobility. As a verb, blacklist can mean to deny someone work in a particular field, or to ostracize them from a certain social circle.

Hollywood blacklist
In computing, a blacklist is an access control mechanism that means, allow everybody, except members of the blacklist. The opposite is a whitelist, which means, allow nobody, except members of the white list. As a sort of middle ground, a greylist serves as a temporary blacklist that could be used, for example, to block poorly-configured electronic mail clients that may be used to send undesirable electronic mail.
An organization may keep a blacklist of software or websites in its computer system. Titles on the list would be banned and everything else would be allowed. For example, a school might blacklist KaZaA and ICQ; other Internet services would still be allowed.
Content-control software such as DansGuardian and SquidGuard may work with a blacklist in order to block URLs of sites deemed inappropriate for a work or educational environment.
An e-mail spam filter may keep a blacklist of addresses, any mail from which would be prevented from reaching its intended destination. A popular technique for implementing blacklists is DNS blacklisting (DNSBL).
Many copy prevention schemes include software blacklisting.

Medical context

James J. Lorence. The Suppression of Salt of the Earth. How Hollywood, Big Labor, and Politicians Blacklisted a Movie in Cold War America. University of New Mexico Press: 1999. ISBN 0-8263-2027-9 (cloth) ISBN 0-8263-2028-7 (paper)
Patient Safety and Blacklisting