The two suprises of today's meetings of President Reagan and Soviet leader Mikhail Gorbachev -- their unexpectedly long private talks and the sudden news blackout -- may be essential ingredients in any substantial results that flow from the first U.S.-Soviet summit in six years.

Guarded comments from both the U.S. and Soviet sides suggested tonight that cultural, civil aviation, consular and other bilateral agreements of modest importance between Moscow and Washington are on track.

But there was no word on whether Reagan and Gorbachev are making any progress toward even a general understanding on the key questions of nuclear and space arms.

The initial private chat of the two leaders, which was planned for 15 minutes and lasted more than an hour, appeared to be a necessary personal and political ice-breaker.

"If I were in charge, that is the way I would do it," said Prof. Marshall Goldman, director of Harvard University's Russian Research Center. "Given the hostility and the names which have been called, such as 'the focus of evil about the Soviets' and 'another Hitler' about Reagan, it is hard to see how they could get anywhere without finding a way to smooth things over."

The second private chat, for nearly an hour more during a walk and a fireside discussion, came at the end of an afternoon meeting on the tough issue of arms control. This suggested that the walk may have been the occasion for the effort that Reagan had spoken of in advance to convince Gorbachev that the "Star Wars" space-based missile defense plan is a boon to peace and stability rather than a threat to Soviet survival.

The results of the late afternoon private discussion were known only to a very few members of the U.S. delegation tonight.

Progress at such a top-level session would seem to be an essential first step if there is to be any accommodation on the crucial issue of space weapons, in view of the fact that no compromise had been worked out on this point in the months-long diplomacy leading up to today's talks.

The news blackout, which came as a surprise to most of the press and to many officials on both sides, was the work of Secretary of State George P. Shultz, who used the same device during his successful Jan. 7-8 talks here with then-Soviet Foreign Minister Andrei Gromyko that restarted U.S.-Soviet arms talks a year after the Soviets had walked out of them.

Secrecy during the course of diplomatic negotiations is not unusual, but the close-mouthed Shultz is believed to have adapted his January procedure from U.S. labor negotiations, at which he had previously demonstrated achievement as a private mediator and as secretary of labor.

A typical negotiating tactic of the Soviets, like some U.S. union and management bargainers, is to insist on rigid stands and impossible demands right up to the final hour. Then, as the pressure mounts almost intolerably, the Soviets give the minimum needed for agreement. In the case of the January meetings, the deal-making final session on the last afternoon was set for three hours and lasted nearly six, in part because of a Gromyko threat to terminate the discussions without agreement if U.S. compromises were not forthcoming.

In such a situation, blow-by-blow press descriptions, whether accurate or not, tend to work against the United States by contributing to public pressure to avert diplomatic failure and make a deal. The Soviets, whose public opinion is more easily controllable but who are at ease with secrecy in all things, do not seem to object to news blackouts.

In the January meetings -- and very likely this time -- the news blackout lasted only until the announcement of results at the end of the two-day meeting.

Shultz's announcement of an agreement and a press conference at midnight on Jan. 8 was attended by senior arms control officials of the White House, State and Defense departments and Arms Control and Disarmament Agency.

In a madcap scene as Shultz finished speaking, scores of reporters buttonholed the officials, now suddenly released from their vows of silence, and a frantic scramble of impromptu news briefings ensued then and there.

The difference this time is a much more sophisticated and vocal Soviet public relations effort, an extraordinary feature of the current Geneva meeting, and the even more intense international press and television spotlight.

There was a danger, due to the combination of those factors, that the cacophony of rival public relations machines here would leave the world with the impression for two days that the Reagan-Gorbachev summit was a dangerous confrontation, no matter what was happening in the confines of the meeting room.