With allusion to the traditional colour of police uniforms, the expression blue flu originated in the USA in the sense of a form of industrial action in which large numbers of police officers fail to attend work on the pretext of illness. This expression later came to also designate industrial action of this type taken by other workers. Such action may be taken to circumvent contractual bans on striking.

—Cf. also the British-English expression man flu, denoting a cold as experienced by a man who is regarded as exaggerating the severity of the symptoms.

An interesting extended use of the expression blue flu occurred in the following from the Daily Record (Glasgow, Lanarkshire, Scotland) of Wednesday 9^th December 1998:

A DOSE OF BLUE FLU
Rangers fans stay off work to watch big Euro clash
By PAUL HARDAKER

HUNDREDS were caught up in a “Blue Flu” epidemic yesterday as football fans went off sick to watch the crunch UEFA Cup clash between Rangers and Parma.
Legions of fans crowded into pubs across Scotland to watch the big match which kicked off on BBC 2 at 1.30.
About 60 Rangers fans packed into The Grapes Bar at Paisley Road West, Glasgow, to watch the game.
Landlord Brian Sproul said: “There’s one or two people gone down with the Blue Flu.
“We’re lucky to get 20 people in on a Tuesday afternoon but we were packed for the match. Most of the people who came to watch the game were kidding on that they were at the doctor’s or dentist.”
One drinker told his boss he wanted the day off to go to his grandmother’s funeral.
He said: “Rangers are my life. I wouldn’t miss the game for anything.
“Over the years, I’ve lost six grannies, 19 grandads and I’ve been for about 45 hospital appointments.”

The earliest occurrences of the expression blue flu that I have found are from reports from Detroit, Michigan, by the Associated Press and by United Press International, published in many newspapers on Friday 16^th June 1967:

1-: From the Associated Press report, published, for example, in The Montgomery Advertiser (Montgomery, Alabama, USA):

DETROIT (AP)—Nearly 200 of Detroit’s 4,000 policemen called in sick on Thursday shifts after some of their number had been disciplined because of a slowdown in writing of traffic violation tickets.
Dubbed “blue flu” by observers, the rash of sick call-ins appeared likely to grow. Top brass said severe steps might be taken to provide a cure.
Commissioner Ray Girardin, head of the Motor City’s blue-coat force, called it a “serious situation” and said he would take “whatever steps are necessary” if conditions did not improve.
The sick call-ins came after Girardin put 42 traffic officers on foot patrol beats, taking them from motorcycle and scout car assignments.
Ticket writing for traffic violations had fallen off an estimated 80 per cent in a month-long slowdown.
The slowdown began after Mayor Jerome P. Cavanagh rejected police pay boost demands on grounds the city budget wouldn’t permit it.
Girardin gave no particulars on the future but indicated he might recommend calling for help from the National Guard if needed.
The commissioner ordered city doctor examinations of policemen reporting ill and said anyone found a “malingerer” would be suspended.
At the same time the policemen’s union attorney, Winston Livingston, predicted 500 men might call in sick by Saturday and 1,000 by Monday.
“It’s going to spread through the department,” said Livingston.
The policemen’s union, the Detroit Police Officers Association, has denounced Mayor Cavanagh’s stand on police pay. Police pay starts at $7,400 a year and advances in steps to a maximum of $8,335 after five years service.
Patrolman Carl Parsell, union president, said the union does not endorse the ticket-writing slowdown.
However, he said policemen had the right to use their own discretion in ticketing motorists and he defended the men who called in sick.
Parsell, 43, a policeman for 21 years, charged Girardin used a “pressure tactic” in the disciplining of the 42 men. He said the sick calls were “triggered” by the fact one of the transferred policemen was a union steward.

2-: From the United Press International report, published, for example, in the Daily World (Opelousas, Louisiana, USA):

DETROIT (UPI)—Detroit policemen went on 12-hour shifts today as hundreds of fellow officers continued a “sick call” strike in an effort to force the city to give them a wage boost.
By early today, 300 policemen of the city’s 4,780-man force had called in sick with what has been nicknamed the “blue flu.” More were expected today. A spokesman for the Detroit Police Officers Association (DPOA) warned the number of sick policemen would jump to 1,000 by Monday.
The Michigan State Police and National Guard were alerted for possible duty if the reported illnesses showed any signs of sharply cutting the effectiveness of the Detroit Police Department.
The Wayne County Circuit Court late Thursday ordered the DPOA to immediately halt what was termed an “illegal and unauthorized strike and work stoppage.” A DPOA spokesman said the ruling would be appealed and said it had upset “a delicate balance in a labor dispute.”
All leaves and furloughs were cancelled by Police commissioner Ray Girardin and men reporting for work were told they would have to put in 12-hour shifts. The normal daily rate of sickness absentees is 52, a police spokesman said.
At the heart of the problem is the DPOA’s demand for a $10,000 a year salary for policemen in the near future.
“The top pay for a police officer with five years service is $8,355 a year,” said Carl Pacell, DPOA president. “We make less than a carpenter’s laborer.”
The city reacted immediately to the high incidence of sick calls. Corporation Counsel Robert Reese filed suit in Circuit Court charging the DPOA with staging an “illegal and unauthorized strike and work stoppage.” The city asked $50,000 in damages per day from the union.
The sick call boycott was the latest in a series of maneuvers by the policemen to back up their wage demands. A ticket-writing slowdown has been in effect for several weeks, resulting in an 80 per cent drop in the number of tickets issued.