After the initial euphoria surrounding this week's agreement to bring black majority rule to Rhodesia, blacks and whites acknowledged yesterday that the negotiations still face formidable obstacles.

Prime Minister Ian Smith and African nationalist leaders did take another step forward as Smith formally agreed on a formula under which black guerrillas now fighting the Smith government would be accepted into the government's armed forces if they so desired.

But Smith's acceptance was conditional on resolution of the still thorny issue of the composition of an interim government to lead Rhodesia to black rule.

It is highly unlikely that the leaders of the black guerrilla forces, which are operating from bases in neighboring Mozambique and Zambia, would be satisfied by the agreement since they have already rejected the Salisbury talks between Smith and locally based nationalist representatives.

Although details of the accord were not disclosed, it appears tha black guerrillas would have to lay down their arms before they could seek to join the government forces.

Yesterday, Smith persuaded the nationalist leaders to hold another closed session today. The hope that this would produce another major advance in the talks overshadowed Smith's acceptance of the integration formula.

Moreover, in recent months Smith has dropped his previous adamant refusal to accept the integration of guerrillas much as he, in the past year, has come to accept the once abhorrent principle of black majority rule. The country's 6.8 million blacks now have practically no voice in the government. There are 263,000 whites.

Conference sources said Bishop Abel Muzorewa's United African National Council presented proposals which called for Smith to step down as prime minister and for the white dominated parliament to be dissolved. In its place would be created a council - involving Smith and the black delegation leaders apparently as equals - which would exercise both executive and legislative power during a transition period.

Although Smith did not present detailed proposals at the 38th conference session, informed sources insisted he had no intention of stepping down. Instead, they added, he wants to retain effective control and appoint the three black delegation leaders as ministers of state without portfolio in a basically unchanged government. Additionally, each black delebation would appoint an apportioned share of what were called "co-ministers" who would work alongside the incumbent whites.

Conference sources refused to speculate how long the negotiations might require to overcome the seemingly diametrically opposed views on the nature and composition of any interim government.

But they noted that Muzorewa abruptly dropped previously vigorous objections to Smith's demands on representation for Rhodesia's whites in a restricted session Wednesday.

The negotiators also agreed on a list of tasks for an interim government to bring about a cease-fire which Smith and his fellow negotiators hope will be heeded by guerrillas fighting for the externally based patriotic Front. Its leaders have denounced the negotiations and sworn to fight on.

The list mentioned "the composition of the military forces, including those nationalist forces who take up a military career, and the rehabilitation of others," the review of political prisoners' sentence, the release of admiinistrative detainees, the removal of racially discrminatory legislation and the registration of black voters and final draft consitution.

Symptomatic of the white business community's reaction to the renewed movement in the negotiations was a statement by Mike Daffy, president of the Associated Chambers of Commerce.

He praised the "new surge of realism and cooperation" and said he earnestly hoped "the Western powers will recognize in the agreement the basis of a just and lasting settlement of the constitutional dispute."

A white airline office employee said "at least they finally got somewhere - that's what the rest of the world wanted," as if to suggest she had her own doubts about the advisability of Smith's policies.

Black public reaction was more circumspect. In private many blacks registered their consternation with Muzorewa, whom they accused of capitulating to Smith only days after he had shown signs of digging in his heeis and demanding concessions from the government.

A messenger in a downtown office asked, "What's in it for the boys," as the guerrillas are often called. "Nothing and that's why it's not going to work."

Even optimistic whites interviewed at random generally feared that a complete agreement on an internal settlement for black majority rule would not end the fighting.