Organized crime has long had a “code of silence,” where one doesn’t testify against another. The New York Police Department also had a “blue code of silence” (named after their blue uniforms). The Knapp Commission (or, Commission to Investigate Alleged Police Corruption) sought to end the “blue code of silence” in its corruption investigations of 1971-72.
 
The term “blue wall of silence” was popularized in 1985 by Queens District Attorney John J. Santucci.
 
 
Wikipedia: B;ue Code of Silence
The Blue Code of Silence (or Blue Wall of Silence) is a supposed unwritten rule among police officers in the United States not to report on another colleague’s errors, misconducts or crimes. If questioned about an incident of misconduct involving another officer (e.g. during the course of an official inquiry), if following the Blue Code of Silence, it would be standard procedure to claim ignorance.
 
Two studies suggest that some police feel that the code is applicable in cases of “illegal brutality or bending of the rules in order to protect colleagues from criminal proceedings”, but not to illegal actions with an “acquisitive motive”.
 
Cases such as the Rampart Scandal and many other police corruption cases demonstrate that blue code culture can extend to cover-ups of other levels of crime, acquisitive or not.
     
Wikipedia: Code of silence
A code of silence is a condition in effect when a person opts to withhold what is believed to be vital or important information voluntarily or involuntarily.
 
The code of silence is usually either kept because of threat of force, or danger to oneself, or being branded as a traitor or an outcast within the unit or organization as the experiences of the police whistleblower, Frank Serpico illustrates. Police are known to have a well developed Blue Code of Silence. The code of silence was famously practiced in Irish-American neighborhoods in Boston, Massachusetts such as Charlestown, South Boston, and Somerville.
 
A more famous example of the code of silence is omerta (Italian: omertà, from the Latin: humilitas=humility or modesty), the Mafia code of silence.
   
Wikipedia: Knapp Commission
The Knapp Commission (officially known as the Commission to Investigate Alleged Police Corruption) stemmed from a five-member panel initially formed in April 1970 by Mayor John V. Lindsay to investigate corruption within the New York City Police Department. The creation of the commission was largely a result of the publicity generated by the public revelations of police corruption made by Patrolman Frank Serpico and Sergeant David Durk.
   
Investigation and public hearings
While the Knapp Commission (named for its chairman, Whitman Knapp) began its investigation of corruption in the police department in June 1970, public hearings didn’t start until October 18, 1971. In addition to the testimony of lamplighters Serpico and Durk, testimony from dozens of other witnesses, including former Police Commissioner Howard R. Leary, corrupt patrolmen and the victims of police shakedowns, were heard.
 
As an immediate result of the testimony of the witnesses, criminal indictments against corrupt police officials were handed down. Commissioner Patrick V. Murphy, who was appointed by Mayor Lindsay shortly after the commission was formed to clean up the department, implement proactive integrity checks, massive transfers of senior personnel, job rotation in key areas, ensuring sufficient funds to pay informants, and cracking down on citizen attempts at bribery.
     
wiseGEEK
What is meant by the Blue Wall of Silence?
The Blue Wall of Silence is the name used to describe the unity exhibited by Police officers in an aim to limit their co-operation when the accused is a police official. It is neither a morally justifiable act nor is it a legal act.
 
Experts say that the Blue Wall of Silence symbolizes the fierce loyalty among police officers. According to NPR’s Eric Westervelt, the idea of the impenetrable ‘code’ came into existence when a Haitian immigrant was tortured in custody at a Brooklyn precinct. In reality, the Blue Wall of Silence was built by Jerome H. Skolnick who reported the case in April 2000.The trial that began on May 4 of four New York policemen in connection with the August 1997 brutalization of Haitian immigrant Abner Louima took two surprising turns. In the second week of the trial, three NYPD policemen took the stand to support the prosecution’s charges that one of the defendants, Officer Justin A. Volpe, had sodomized Louima with a wooden stick in the restroom of the 70th Precinct station house in Brooklyn. The three cops, breaking through the notorious “blue wall of silence,” testified that Volpe had boasted of the crime, and they offered accounts supporting aspects of the charges. The fact that three cops had testified against another cop in a police brutality case took the media by storm. Then, the second surprising turn was that, Volpe decided to plead guilty to the charges. This confession shocked the community at large who felt that these killers in blue uniforms cannot be reformed but must be removed from our communities. Media reports said that the three cops who testified against Volpe were emboldened to cross the “blue line of silence” by the mass protests against police brutality that followed the murder of Amadou Diallo. Thus the Blue Wall of Silence came into existence.
     
7 August 1972, New York (NY) Times, “‘Meat-Eaters’ and ‘Grass-Eaters,’” pg. 32:
Following are excerpts from the first section of the Knapp Commissions final report—a summary of general findings and recommendations—that was released yesterday.
Corruption, although widespread, is by no means uniform in degree. Corrupt policemen have been described as falling into two basic categories: “Meat-eaters” and “grass-eaters.” as the names might suggest, the meat-eaters are those policemen who, like Patrolman William Phillips, who testified at our hearings, aggressively misuse their police powers for personal gain. The grass-eaters simply accept the payoffs that the happenstances of police work throw their way.
(...)
And yet, grass-eaters are the heart of the problem. Their great numbers tend to make corruption “respectable.” They also tend to encourage the code of silence that brands anyone who exposes corruption a traitor. At the time our investigation began, any policeman violating the code did so at his peril.

31 December 1972, New York (NY) Times, “Knapp Conclusion Warily Optimistic” by David Burnham, pg. 23:
The Knapp Commission has expressed guarded optimism about the chances that the corruption it said infected the Police Department in 1971 can be considerably reduced.
(...)
A Beginning Made
The second tradition, less pervasive, is the “code of silence,” once honored by those in the department who were honest as well as those who were corrupt, which “sometimes seemed to mark the reporting of corruption more heinous than the practice of corruption.”
 
25 April 1985, New York (NY) Times, “2 More Officers Charged in Inquiry Into Torture at a Queens Precinct” by Selwyn Raab, pg. A1:           
At the City Hall news conference, John J. Santucci, the Queens District Attorney, said a lack of cooperation by some officers in the precinct was impeding his investigations into the brutality complaints.
 
“Blue Wall of Silence”
“There is a difficulty in getting people to come forward and talk to us,” Mr. Santucci said in an interview after the news conference. “There is some degree of a blue wall of silence, the so-called code of silence. But there are cracks in the wall, some penetration, and some officers have come forward to tell us things.”
   
New York (NY) Times
SANTUCCI SAYS SOME OFFICERS ARE AIDING TORTURE INQUIRY
April 29, 1985
[ DISPLAYING ABSTRACT ]
Some cracks’’ have developed in a ‘‘blue wall’’ of silence by police officers who have refused to cooperate with an inquiry into charges that suspects had been tortured with electric-shock devices, according to the Queens District Attorney. The District Attorney, John J. Santucci, said in a television interview broadcast yesterday that one officer had told investigators that there had been a stun gun at the 106th Precinct station house, and that another had told of a retirement drinking party there on April 17, the day one suspect said he had been tortured.
 
New York (NY) Times
NEW YORK; WHEN SILENCE IS POISON
By Sydney H. Schanberg (The New York Times); Editorial Desk
May 25, 1985, Saturday
Late City Final Edition, Section 1, Page 23, Column 4, 754 words
[ DISPLAYING ABSTRACT ]
It’s called the ‘‘code of silence’’ or the ‘‘blue wall of silence.’’ It is the corrosive tradition inside police departments that secretly decrees that cops do not turn in, or testify against, other cops who are guilty of misconduct or crime. We are hearing about it in New York City these days because many officers have turned mute in cases ranging from a hit-and-run incident in which a police car killed a pedestrian to a scandal in a Queens precinct where electric-shock torture was allegedly used to force confessions out of suspects. Covering up for one’s fraternity brothers is not unique to the police. Doctors have been known to draw their wagons into a circle to protect errant colleagues. The same is true of lawyers, teachers and, yes, journalists.
 
New York (NY) Times
NEW YORK; THE CONCRETE WALL OF SILENCE
By Sydney H. Schanberg (The New York Times); Editorial Desk
June 25, 1985, Tuesday
Late City Final Edition, Section A, Page 27, Column 1, 762 words
[ DISPLAYING ABSTRACT ]
We’ve all heard of the Blue Wall of Silence, the code under which cops stay mum about rogue cops who poison the barrel. But not much is said of the Concrete Wall of Silence, the code under which real estate developers and contractors stay mum about the corruption in the construction industry.
 
Google Books
Close Pursuit:
A week in the life of an NYPD homicide cop
By Carsten Stroud
New York, NY: Viking
1987
Pg. 193:
The phrase “a blue wall” of silence got headlines.
   
New York (NY) Times
Blacks Fight for a Place Among the ‘Finest’
By DON TERRY
Published: November 19, 1988
(...)
In the early 1980’s, several Guardians Association members testified at a series of hearings on police brutality in New York, breaking the so-called blue code of silence.
   
Google Books
Buddy Boys:
When good cops turn bad
By Mike McAlary
New York, NY: Charter Books
1989, ©1987
Pg. 136:
He would not crack the Blue Wall of Silence. Rats did that. And rats were bad for morale.
 
“I just can’t turn,” Brian said later. “The Blue Wall. You don’t tell. I couldn’t tell. I’d eat the gun first.”