Negotiators for the National Football League and the striking NFL Players Association met briefly just before midnight tonight with newly designated mediator Sam Kagel, who immediately announced he was imposing a news blackout for the duration of the talks.

"As a condition of this mediation I have asked all parties, players and owners throughout the league, to make no public comment concerning the mediation efforts while the mediation is ongoing," Kagel said. He refused to answer any questions.

Kagel, 73, a San Francisco lawyer and veteran West Coast labor arbitrator and mediator, was chosen by Kay McMurray, director of the Federal Mediation and Conciliation Service, to mediate the 22-day-old strike.

Kagel called the parties together at a suburban Baltimore inn tonight for their first full-scale negotiations since the sides broke off Oct. 2 after three days of fruitless talks in Washington.

Kagel arrived at the Hunt Valley Inn shortly before 11 p.m. "My tactic will be to tire them out before they tire me out," he said.

Kagel met briefly with Ed Garvey, executive director of the NFLPA; union President Gene Upshaw, of the Los Angeles Raiders; NFL chief negotiator Jack Donlan and Sargent Karch, general counsel to the NFL Management Council, the league's labor negotiating arm, before imposing the news blackout.

McMurray's selection of Kagel followed a five-hour meeting Monday between McMurray, Garvey, Upshaw, Donlan and Karch. McMurray called Kagel "a prominent private mediator and arbitrator . . . and the most acceptable choice to both sides as mediator."

He said Kagel will have control of the mediation and will schedule all meetings. But Kagel will keep McMurray updated on the progress or lack of it in the talks.

Kagel, who has mediated and arbitrated thousands of labor disputes, is known in his field as the developer of mediation-arbitration, a process in which an outsider mediates a dispute but then issues binding arbitration if necessary. Both sides in this strike have said they are opposed to binding arbitration.

Kagel is a member of a panel that arbitrates noninjury NFL grievances and has arbitrated two grievances filed by former Baltimore Colts quarterback Bert Jones against team owner Robert Irsay. Last April, Kagel ruled against allegations by Jones that Irsay reneged on a verbal agreement to pay him $750,000 and made derogatory statements about him during 1981 contract negotiations.

According to The Associated Press, Kagel has developed a reputation for fairness that has kept him in demand as an arbitrator and mediator since he started his own law firm in the late 1940s.

"I won't play footsie with either side," Kagel said while explaining his philosophy in a 1971 interview with the San Francisco Examiner. "I try to find out what each side wants--not what they think they want or what they say they want, but what they really want and need--and then I try to get it for them."

Within minutes after Kagel's appointment as mediator was announced, the NFL formally announced that games scheduled for this Sunday afternoon and Monday night had been called off. Since only two weekends' games can be made up, that means that each team will play at least two fewer games this season, an NFL spokesman said.

At an afternoon news conference today, Garvey and Upshaw said they welcomed Kagel's participation in the negotiations, but cautioned against attaching too much significance to that development.

"We're pleased that the mediator has been selected. We think he's an excellent choice," Garvey said. "But we see this as the beginning of negotiations, not the end of the strike."

Kagel, Garvey said, "is familiar with our industry. He has been a neutral arbitrator under our grievance procedure."

Garvey and Upshaw also predicted that the league increasingly will talk of cancelling the season if the strike isn't settled soon. But, said Garvey, "They would hold the season if there were one game left."

Upshaw also argued that the union has the right to negotiate how many games will be played in a strike-abbreviated season, and that the decision is not the league's alone.