The House Assassinations Committee appointed a former government prosecutor as its new chief counsel yesterday and then procliamed a gag rule on its investigations into the murders of President Kennedy and the Rev. Martin Luther King Jr.

The new counsel, G. Robert Blakey, 41, currently director of the Cornell Institute on Organized Crime, vowed to make the news blackout stick and said he hoped the press would simply go away until the investigations are completed.

"The purpose of this news conference is to announce there will not be any more news conferences," Blakey told reporters yesterday afternoon following announcement of his appointment by Chairman Louis Stokes (D-Ohio).

Known as a hard-line expert on criminal law, Blakey served as a special attorney in the organized crime and racketeering section of the Justice Department from 1960 to 1964. Inter, as a special consultant to the Senate Judiciary Subcommittee on Criminal Laws and Procedures, he played a major role in drafting Title III of the Omnibus Crime Control and Safe Streets Act of 1968, the section that authorized government wiretapping and bugging with judicial warrants.

Rep. Christopher J. (Dodd (D-Conn.), who headed a special committee task force to find a successor to former chief counsel Richard A. Sprague, said the members sifted through 115 possible candidates for the job, selected 34 for prospective interviews and actually interviewed 13.

"Blakey was our principal choice," Dodd said, adding that the committee began dickering with the Cornell University law professor about a month ago.

Former Watergate special prosecutor Archibald Cox and former Supreme Court Justice Arthur Goldberg had turned down earlier overtures to take the assignment.

Sprague quit March 29 after a prolonged dispute with former committee Chairman Henry B. Gonzalez (D-Tex.) over the staffing and spending for the inquiry.

Stokes coupled announcement of Blakey's appointment with promulgation of a new committee rule declaring that "all information" will be kept confidential "during the preliminary evidence-gathering phase" in order to protect the witnesses and the "integrity of the process of the investigation."

"As to the substance of this investigation," the resolution stated, "it shall be the policy of this committee and its staff not to comment until this investigatory phase is completed."

Blakey began his brief remarks by quoting the words of Thomas B. Dewey in a 1925 radio address shortly after his appointment as a special prosecutor in New York City to investigate organized crime.

"In general, it is my belief that a talking prosecutor is not a working prosecutor," Blakey approvingly quoted Dewey as saying. "'It is my sincere hope that the work we are doing vanishes from the newspapers.'"

Admittedly ill at ease through the rest of the press conference, Blakey avoided direct answers to many of the questions put to him, declining to say when public hearings might be expected and what issues involving the two assassinations he thinks it most important to resolve. He said his $47,-[WORD ILLEGIBLE] congressional salary would leave him with a substantially reduced income flow. When asked why he took the job, Blakey said tersely: "I think it's important that it be done."