The Carter administration, in the first major break in its 3-months-old moratorium on high-level official trips to the Soviet Union, has approved a forthcoming mission to Moscow by Treasury Secretary W. Michael Blumenthal.

The moratorium, which applied to trips by policy-level officials except for those on arms control business, was ordered in early July, and led to cancellation of several high-level journeys. The purpose of the travel curb was to show U.S. displeasure with the trials of Soviet dissidents and legal proceedings against American journalists and businessman.

Approval of the Blumenthal trip in early December for a U.S.-Soviet trade conference was confirmed by Treasury Department officials. The trip is reported to have been among the subjects discussed yesterday in a meeting of President Carter, Blumenthal and Michael Forrestal and William Verity, officials of the U.S.-U.S.S.R. Trade and Economic Council.

Approval of the Blumenthal trip, which is certain to be taken as a favorable omen by Soviet leaders came as Moscow radio and television broadcast a lengthy and unusually detailed interview with Foreign Minister Andrei A. Gromyko, giving a positive interpretation of his meeting at the White House a week ago.

Speaking of the proposed strategic arms limitation treaty (SALT), the central point in his discussions with Carter and Secretary of State Cyrus R. Vance, Gromyko said "there has been certain progress in the required and correct direction" on the part of the United States. In another remark, Gromyko, called the U.S. SALT proposals "realistic," and said he hoped they would prevail against some opposition in Washington.

Gromyko said the U.S. shifts are "not sufficient to enable me to say that the questions have already been settled," but he referred expectantly to the planned meeting with Vance and a U.S. SALT negotiating team in Moscow later this month.

Gromyko's hour-long interview set the stage for the long-expected summit meeting of Carter and Soviet leader Leonid L. Brezhney by informing the Russian people of this growing possibility. Gromyko said he had informed Carter that Brezhnev was prepared to meet him if "something considerable and important" such as the SALT agreement could be "confirmed and signed" at the time. He said Carter agreed that the summit meeting should be "well-prepared."

Saying that the documents for meeting Gromyko seemed to be sig-SALT should be prepared before the naling more clearly than before that the Carter-Brezhnev summit should not be a forum for major bargaining. Administration officials have interpreted such remarks previously made in private as motivated by concern over Brezhnev's health.

In the Soviet view a Carter-Brezhnev meeting should be the occasion for a new flowering of detente after a period of chill and at times open hostility.

Gromyko said he had "quite bluntly" spelled out for Carter that "it is not our fault, it is the fault of the United States" that the two nations had been in "an unsatisfactory situation" in recent months.

"We quoted concrete examples confirming this premise, but they told us in reply: 'You see, maybe it would be best not to go into the past and to look ahead,'" said Gromyko. According to reports from Moscow, he then chuckled and said it is understandable to want to look ahead, but "you cannot put up an insulating wall between the past and the future."

The Gromyko Interview expressed Soviet displeasure at the Camp David Mideast agreements, but in milder terms than the harsh remarks by Brezhnev at a dinner for Syrian President Hafez Assad, also reported in Soviet news media yesterday.

In view of the crosscurrents in relations with Moscow, American officials are more reticent than Gromyko in holding out a possibility of a new era of detente.

The approval for the Blumenthal trip, while described as a major step, is described as a "case-by-case" decision rather than a sign of a sweeping revision of policy.