A press blackout in the baseball negotiations was imposed yesterday at the request of Secretary of Labor Raymond J. Donovan.

Federal mediator Kenneth E. Moffett said, "The main reason for the blackout is the same reason we came to Washington. We thought it would be good if they could talk back and forth" without coming down every day "to report what's going on like a tennis match . . .

"Basically, we were in a goldfish bowl up in New York," he added. "We thought it would be a good thing if they could talk back and forth without expalining away what happened upstairs."

The talks, held for the first time in Washington at the Federal Mediation and Concilation Service, were recessed after the parties had met separately and together for about four hours. They will be resumed at 9:30 a.m. today.

Moffett said that Donovan, who participated in the meetings all yesterday, will return to the bargaining table today. "The secretary and the administration are very interested in getting baseball back in business again," Moffett said.

However, he said, Donovan had offered no proposals yesterday that might bring an end to the 39-day-old strike.

Moffett declined to say whether progress had been made or to characterize the tenor of the talks.

However, there was no reason to believe, based on Moffett's remarks, that the imposition of the blackout meant that a settlement was imminent or even that progress had been made. There was no indication that the tension that surrounded the negotiations when they were recessed Thursday had dissipated or that anything other than a feeling-out process had taken place.

Moffett alluded to the reasons for the blackout last week when he spoke about how the parties begin each session by talking about what was said to what reporter in the newspaper that morning.

The implication is that the blackout will both save time and reduce the level of acrimony. Although Judge Henry F. Werker said last month, in his decision on the National Labor Relations Board request for an injunction, "This court cannot accept collective bargaining through the press as a basis for an injunction," the press has played a significant role in the talks. Sources have said that previous initiatives in the talks have failed or been aborted because of disclosures in the press.

Moffett said last week, "Any time you have to a total disclosure with the media, you're going to have difficulties. There's going to be posturing . . . Only time will tell if this mode of bargaining is one that leads to a settlement."

Now, only time, perhaps three days, will tell if a blackout will lead to a settlement. Undoubtedly, if it does not, a lot of people will have a lot to say.

Right now, the parties are respecting Donovan's request. None of the principals reached last night had any comment, on or off the record.

Donovan, who was expected merely to start yesterday's meeting, met separately with each side for about 45 minutes, before bringing them together for joint negotiations. Donovan met first with the owners' bargaining committee, including Ray Grebey, their chief negotiator, his attorney Barry Rona, the two league presidents and their attorneys.

The players were represented by Marvin Miller, executive director of the Major League Baseball Players Association; Don Fehr, the general counsel, and five players: Mark Belanger and Doug DeCinces of the Baltimore Orioles, Steve Rogers of the Montreal Expos, Bob Boone of the Philadelphia Phillies and Phil Garner of the Pittsburgh Pirates.

The parties met together with Donovan for about an hour in the suite where the air traffic controllers reached their tentative settlement, now in jeopardy.

Before the talks recessed for the day, Nancy Broff, the acting general counsel of the Federal Mediation Service, said, "The secretary is doing a lot of listening and some talking. The secretary is letting Ken Moffett do the mediating. He is not attempting to insert himself as a mediator."

Donovan first entered the negotiations last Wednesday, when he met with both sides in New York.

But that day the tenor of the talks degenerated when a proposal expected from management failed to materalize and the players left the session feeling angry and puzzled, Thursday, both sides made and rejected proposals. And at the end of the day, the players proposed submitting the issues to binding arbitrtion and the owners rejected that idea.

Donovan met privately Friday with Miller and Grebey and suggested moving the talks to Washington. It is understood, as reported last week, that he suggested the sides deal with free-agent compensation before taking up the major league service credit issue introduced by the owners Thursday.

Private and federal labor officials speculated that Donovan, who steadfastly had said he would not become involved in contract talks, made an exception for the baseball strike at a time when many were optimistic about a quick settlement, thinking it would end quickly and happily for all involved, including his office. But, as one source said, when Donovan got involved he found out "it wasn't just a game."