Allan E. Blanchard, 49, chief of the Washington bureau of The Detroit News, died yesterday at George Washington University Hospital where he had been under treatment for Legionnaire's disease.

Mr. Blanchard, who had worked for The News for 24 years, covering a wide variety of major stories at home and abroad, was admitted to the hospital Aug. 14, but did not respond to treatment, according to a nursing supervisor.

Mr. Blanchard was one of two persons reported earlier this month to be under treatment in the District of Columbia for Legionnaire's disease - a form of pneumonia. The second man was reported yesterday to be still under treatment in the intensive care unit at the Washington Hospital Center. His condition was described as serious but steadily improving.

No other persons here are known to be suffering from the disease, first diagnosed after 29 persons attending an American Legion convention in Philadelphia in 1976 died from a then-mysterious cause.

Since that time a number of cases have been reported around the nation and abroad. Five cases, including two deaths had previously been reported here.

Dr. Martin Levy, chief of communicable disease control for the District of Columbia, said he had no idea how Mr. Blanchard contracted the illness. "We wish we knew that," he said. "You never know on these isolated cases."

An award-winning journalist, Mr. Blanchard was known in the 1960s as his paper's top general assignment reporter, and covered such stories as the six-day war in the Middle East, the civil rights movement in the South, and the trial of Jack Ruby for the shooting of Lee Harvey Oswald.

He was caught in a firefight in 1965 between American forces and rebel snipers during the United States intervention in the Dominican Republic.

After beginning a column in 1968 in which he showed an ability to treat new topics with wit and whimsy as well as seriousness, he became national editor in 1970. Mr. Blanchard came here as bureau chief in 1975.

Mr. Blanchard, an avid trombonist, was a graduate of Wayne State University in Detroit, where he studied music and journalism.

In addition to his wife, Shirley, survivors include two sons, Stephen and John, of the home in McLean, his father and stepmother, Mr. and Mrs. Frank E. Blanchard, of Grosse Pointe, and a brother, Donald, of Grand Rapids.