A blacklist (or black list) is a list or register of persons who, for one reason or another, are being denied a particular privilege, service, mobility, access or recognition. As a verb, to blacklist can mean to deny someone work in a particular field, or to ostracize a person from a certain social circle. Conversely, a whitelist is a list or compilation identifying persons or organizations that are accepted, recognized, or privileged.
According to the Henry Holt Encyclopedia of Word and Phrase Origins the word "blacklist" originated with a list England's King Charles II made of fifty-eight judges and court officers who sentenced his father, Charles I, to death in 1649. When Charles II was restored to the throne in 1660, thirteen of these regicides were executed and twenty-five sentenced to life imprisonment, while others escaped.
They also point out that "white list" is not the opposite of a blacklist, but rather a list, often kept by unions, of people suitable for employment.
The term blacklisting is generally used in a pejorative context, as it implies that someone has been prevented from having legitimate access to something due to the whims or judgments of another. For example, a person being served with a restraining order for having threatened another person would not be considered a case of blacklisting. However, somebody who is fired for exposing poor working conditions in a particular company, and is subsequently blocked from finding work in that industry, may be considered to have been blacklisted. Blacklisting can and has been accomplished informally and by consensus of authority figures, and does not necessarily require a physical list or overt written record.
In American history, one of the most famous examples of blacklisting stemmed from an investigation launched in 1947 by the House Un-American Activities Committee (HUAC) into Communist influence on the motion picture industry. The first in the industry to be blacklisted, as a result of their refusal to provide evidence to HUAC, were a group known as the Hollywood Ten, most of them screenwriters, who had at one time or another been members of the American Communist Party. Today, the best known of the Hollywood Ten is writer Dalton Trumbo, who was barred from openly working in Hollywood for over a dozen years as a result of his defiance of HUAC. (He continued to work under pseudonyms and "fronts" until the revelation in 1960 that he had written the script for Spartacus.) Actor John Garfield was one of the more famous Hollywood performers to have been blacklisted by major American film studios as a direct result of HUAC investigations and hearings.
Blacklisting may sometimes result in a domino effect, as in the case of radio actress Madeline Lee. Both Lee and her husband, actor Jack Gilford, were blacklisted during the McCarthy era after it was revealed that Lee had given a party in her house to raise funds for a group later labeled as a Communist front. Though there was no suspicion that she had ever been involved in any putatively "subversive" political causes (and though her real name was spelled differently), Lee became the target of thousands of protest phone calls to her network. Another actress, Camilla Ashland, who appeared on the television show Danger, physically resembled Madeline Lee; though she had no political past, her network too became the target of protest phone calls. Madeline Pierce, a 20-year veteran of radio, who again had no political past, was also ultimately blacklisted.
Following the passage of California's Proposition 8, Proposition 8 opponents obtained donation lists of those who had supported the ballot measure by contributing to the "Yes on 8" campaign, published the list, organized an activism group, and began calling for boycotts of the places of work of the supporters to force the firing or resignation of employees. Chad Griffin, a political adviser to Hollywood executives and same-sex marriage supporter explained the intent of the campaign by saying, "Any individual who has held homophobic views and who has gone public by writing a check, you can expect to be publicly judged. Many can expect to pay a price for a long time to come." There has been controversy about whether this campaign is an appropriate response to the passage of Proposition 8 by those who opposed it.
Trade union members in the United Kingdom have been blacklisted by employers using the services of the Economic League (UK) which operated between 1918 and 1993, and The Consulting Association which took over this role until it was closed down in February 2009.
On October 16, 2008, Capitol West Appraisers filed a class action lawsuit against Countrywide Financial Corp. (now owned by Bank of America) alleging Countrywide blacklisted appraisers who refused to appraise real estate at inflated prices.
In Capitol West v. Countrywide Finacial Corp., Capitol West Appraisals alleges Countrywide pressured appraisers to set the value of a property at the selling price rather than setting the value based on an inspection of the property and comparable market prices in the area as required by appraisal standards. Misrepresenting the value of a property allowed Countrywide to lend more money to more buyers at higher risks with less collateral. Countrywide could then make more money off the loans by selling them on secondary markets as mortgage backed securities.
Appraisers who refused to misrepresent property values were put in a "do not use" or "Field Review List" data base, in essence a blacklist. When an appraisal was submitted by an appraiser on the "do not use" database it was flagged for "field review". A field review is a review of the original appraisal. All field reviews were conducted by Landsafe, a subsidiary of Counrtywide, who would consistently shoot holes in any appraisal from a blacklisted appraiser. Mortgage Brokers were then required to get a second appraisal from another appraiser. Since independent mortgage brokers did not want to pay for two appraisals and did not know which lender they would ultimately use, and since Countrywide was the biggest home mortgage lender in the U.S., brokers simply would not use anyone on the Field Review List.
As of Aug. 28, 2008, more than 2,000 names were on the Field Review List, Countrywide's blacklist, which it sends to mortgage brokers who hire appraisers across the United States. 
In computing, a blacklist is an access control system which denies entry to a specific list (or a defined range) of users, programs, or network addresses.
Blacklisting is multiple providers denying care to a certain patient or patients with a connotation of volition or willfulness. It is done in various ways for various reasons and is not new. In 1907 the Transvaal Medical Union in South Africa blacklisted patients if they could not pay cash in advance. That was a physical list kept by the community of physicians. A physical list is not necessary to blacklist patients, but there have been other efforts to do that. For instance, in the United States the web site http://www.doctorsknow.us was set up to blacklist any patient who had filed a suit against a physician. That effort was extended off shore to a website that encourages doctors to consider avoiding patients who are listed in their database. Those both are physical lists that blacklist patients who either have complained or sued their healthcare providers.
There are less formal and less visible blacklists as well. For instance, an organization called "Sufferers of Iatrogenic Neglect" knows of 40 cases where patients claim they have suffered on two counts: one, from the original human medical error, and two, because they complained about it and as a result got blacklisted. In West London, Rafat Saeed had difficulty finding a GP and says, “… it is very easy for a doctor to blacklist a patient through the Family Health Services Authority”. Angelique Omega wrote in her blog, "I was once told in a phone call by a Renown E.R. nurse, after she very quickly looked up my name in their computer, that I'd better not ever show my face there ever again. This was after I had filed complaints …"
One patient created a graph showing that every time his primary care physician knew about appointments he had with other physicians, those appointments did not result in diagnosis or treatment. All those physicians pretended to be helping, but eventually workers in one physician's office let him know that his primary care physician called them and told them not to diagnose or treat his injuries. They were protecting the physician who caused the injuries.
Data sharing also can cause patients to become blacklisted. Data sharing makes it easy to get labeled as a "problem patient" without anyone adding a name to a list. Repeat patients who are misdiagnosed or undiagnosed, or patients with chronic conditions or mental illness, can get labeled as "problem patients" in computer systems such as HealthConnect or Epic that hold the records of patients and can make it increasingly difficult, if not impossible, to get appointments for care. Such systems have no borders making this a global problem. 
Even without data sharing, collegial loyalty, watching each other's backs, can be enough to result in the denial of care to certain patients. Consider the patient who has been injured by a healthcare provider. Patients with iatrogenic illnesses often cannot get a record made of their injuries and often cannot get treatment. Trudy Newman in her article "Deadly Medical Practices"  described the cause as being physicians having a stronger allegiance to each other than to their patients. They are reluctant to acknowledge the existence of iatrogenic injuries by diagnosing or treating them. A patient with iatrogenic injuries can go from doctor to doctor to doctor without getting diagnosed or treated and never know why. Without a list or any communication between physicians, collegial loyalty by itself results in patients with certain kinds of problems being blacklisted.
However, the term blacklist does connote volition or willfulness. A new and unrecognized disease resulting in patients being unable to find treatment might not be considered blacklisting unless inclination or personal belief or the equivalent had to do with why treatment either was not found or was unreasonably difficult to find.