It will be "a black day" for Israel and for the United States if there is "a real confrontation" between them over the terms of a Middle East peace settlement, former Israeli Foreign Minister Yigal Allon said yesterday.

To avert that risk, Allon urged "a cooling-off period" in negotiations for recovering a Geneva conference between Israel and the Arab nations.

Allon was deputy prime minister and foreign minister in Israel's former Labor government. In talks last week with Secretary of State Cyrus R. Vance and National Security Affairs Adviser Zbigniew Brzezinski. Allon said he found them "rather optimistic" about prospects for opening the Genva talks. But Allon expressed regret at the emphasis he found in Washington on procedure, as compared to the substance of negotiations.

It was encouraging to him, therefore, Allon said, to hear Braezinski say in a television interview Sunday on "Face the Nation" (CBS. WTOP) that the December target date for the January "or even to February" This could permit more time to coordinate policy on the substance of the talks. Allon suggested.

State Department spokesman Hodding Carter III, however, said yesterday that although the conference date "may slip" the Carter administration is "still hoping, seeking, expecting that there will be a conference in December."

What Brzezinski was saying. Carter said, eas only that "a possibility" exists that the target date may not be reached.

An American Israeli defense treaty may be sought as one means of providing s U.S. guarantee to Israel that its security will be safeguarded, Vance said.

That possibility, rasied in earlier administrations was mentioned in a Vance interview with U.S. News and World Report. Vance noted that the United States and the Soviet Union in their Oct. 1 statement of principles for a Middle East settlement expressed willingness to consider "guarantees" for an Arab-Israel peace settlement.

But Vance virtually ruled out stationing American troops in the Middle East as a peacekeeping force as "unlikely and probably unwise."

It has been "the general feeling of most nations," he noted, that neither the United States nor the Soviet Union should have "actual troops" in peacekeeping operations. "Supplying peacekeeping forces is better done by medium and smaller countries," Vance said.

Spokesman Carter said that an American-Israeli defense treaty is "one possible eventually" as a form of future guaratee, but it is not now "an active question of policy."

Allon's concern about an American Israeli "confrontation" was expressed in a breakfast meeting with reporters sponsored by Foreigh Policy magazine, and amplified in a speech to the general council of the World Jewish Congress at the Capital Hilton botel Nahum Goldmann. 82-year-old co-founder of the congress, is retiring as the group's president after a nonconefrmist career in which he has criticized various Israeli governments for inadequate flexibility in seeking peace with the Arabs.

At a time when many American Jewish leaders are expressing apprehension about U.S. pressure on Israel over the format for negotiations in Geneva, Allon told the congress that Israel cannot bow to any "unjustified demand." However, he said, "we have to argue in a way so to enable the dialogue to continue."

Allon said that "if we do not manage to achieve a certain understanding with our friends," especially in the United States, "we may be 'trapped' in Geneva" between adversaries.

"If a confrontation takes place," he said, ". . . this will be a black day in the history of both countries." To stumble into a confrontation "because of misunderstandings," Allon said, would be the worst fate of all. "If the confrontation is to take place," Allon told World Jewish Congress delegates, "let it be on a very crucial problem which all of us can unite around."

Israel, he said, must be able to show its people and the world that it did its utmost to achieve a compromise, whatever the outcome, for "when we have to deal with democracies, public opinion will be crucial."