Computing

In computer systems, a blacklist is an access control system that denies a list of users, websites, or programs permission to access specific data, or prevents them from being accessed themselves.

Medical

In a medical context, a blacklist is a number of people who are denied medical care by multiple physicians, generally because the persons in question are known to be problem patients, or have previously filed a suit against a doctor.

Politics

Politically, a blacklist is made up of those who have been denied access to something, generally because of the prejudices or opinions of another group. Such a list, together with a medical blacklist, is more likely to be theoretical rather than physical in nature.

Retail

Many stores in the United States have a policy of refusing service to those who excessively return products previously purchased. This is known as a retail blacklist.

Differentiated from a Boycott

A blacklist is usually the efforts of a third party to prevent the interaction of, or a transaction between, two others. A boycott, on the other hand, is the decision of a direct party to such an interaction refusing to participate.

Source:

Blacklist Defined

Blacklist on Wikipedia