To the black-ruled nations perched uneasily on South Africa's borders, the Reagan administration's foreign policy in this volatile region appears to have come unhinged with last week's embrace of Angolan rebel leader Jonas Savimbi.

On the Indian Ocean side of southern Africa, Washington is now supporting the Marxist-oriented government of Mozambique, which is battling anticommunist guerrillas. Administration officials have justified this as part of its policy of "constructive engagement" with all the states of southern Africa.

But on the Atlantic side, "constructive engagement" has given way to anticommunist fervor. Washington has decided to aid rebel leader Savimbi, who also gets military aid from Pretoria, in his efforts to overthrow a Marxist government that bears a striking resemblance to that of Mozambique.

While in Mozambique the administration has committed more than $5 million to rebuild a railway line that is vital to regional transportation, in Angola it plans to help rebels who have destroyed another key regional railroad.

The seeming contradiction is baffling and frightening to the region's black governments. Having stood by helplessly as the government of Lesotho, one of their number, fell two weeks ago under heavy South African pressure, many fear that they may be next and that the United States will do little or nothing to protect them.

Their fears and confusion were on display last week at a conference here of nine southern African states -- Mozambique, Angola, Botswana, Swaziland, Lesotho, Zambia, Tanzania, Malawi and Zimbabwe -- with 37 other nations, including the United States, to discuss how to wean this region from economic dependence on its white-ruled neighbor.

South Africa prefers to view itself as the region's economic big brother, but its black neighbors see it as having a stranglehold on their economies. Nearly 80 percent of trade in the region's five landlocked states flows through South African ports, railways and roads, and three rely on South Africa for all of their oil supplies. It also supplies work for migrants.

Although little was said publicly, some delegates clearly were upset that U.S. representatives were discussing economic development here while President Reagan was extending a warm welcome to a rebel movement that has destroyed a significant portion of the region's infrastructure of roads and railways.

U.S. support for Savimbi, said Simba Makoni, the regional official who organized the gathering, was "misdirected and very costly both for our region and for the people of Angola."

Makoni is executive secretary of the nine states' Southern Africa Development Coordinating Conference. He said support for Savimbi's movement would only prolong the bush war that has impoverished Angola and undermined regional development for more than two decades.

"There will be no development, no stability, no western democracy, no free-enterprise system to talk about," said Makoni of the stated U.S. goals. "It will only result in further suffering for the poor people of Angola."

The administration has told congressional intelligence committees it is considering an initial outlay of up to $15 million in covert aid to Savimbi's National Union for the Total Independence of Angola, while conservative Republican lawmakers are pushing for $27 million in overt aid.

The Angolan representative here, Pedro de Castro Van-Dunem, Cabinet minister in charge of energy and a senior member of the ruling Politburo, said his government estimated that Savimbi's rebel movement and periodic South African military operations inside Angola had cost more than $10 billion in ruined infrastructure during the past decade.

The Reagan administration contends that it seeks to play the role of "honest broker" in the region, promoting dialogue between South Africa and its black neighbors and encouraging both sides to talk out their disputes.

Assistant Secretary of State for African Affairs Chester A. Crocker became unpopular early with the proposal that independence for the South African-controlled territory of Namibia be linked to the withdrawal of Cuban troops from neighboring Angola. Many African leaders would like to see the Cubans quit Angola but view the linkage as injecting cold war issues into the matter of independence for Africa's sole remaining colony.

Despite those objections, many southern African states gradually came to accept Crocker's claim that the administration was seeking warmer ties around the region. The developing entente between Washington and Mozambique, along with Crocker's efforts to negotiate a peace settlement between Angola and South Africa, persuaded many to accept his sincerity, if not his policy.

But many Africans say aid to Savimbi runs sharply counter to Crocker's claimed policy and threatens to damage American standing here. Zimbabwe's Ministry of Foreign Affairs issued a weekend statement saying Savimbi's visit to the White House "must, once again, call into doubt the sincerity and, indeed, the morality of those within the present American administration who claim that their only goal in southern Africa is the achievement of peace, justice and equality for all."

The conference delegates reportedly had agreed to avoid this subject. But strong denunciations are likely to be heard Monday, when the region's foreign ministers meet in Lusaka, Zambia.

The one official to raise publicly the charge of U.S. collusion with South Africa was conference chairman Peter Mmusi, one of the most conservative black leaders here. Mmusi is vice president of Botswana, one of Africa's few multiparty democracies, a nation that is under growing pressure from neighboring South Africa to prevent infiltration by black South African rebels seeking to overthrow white rule.

American diplomats here argued that what are known as the "front-line states" would do better to use their influence to persuade the Angolan government to negotiate a peace settlement with Savimbi's UNITA rather than criticizing the United States.

"The Soviets pour in $2 billion in weapons to Angola and no one here criticizes them for interfering, yet when we talk about giving a token sum to a legitimate nationalist movement, we get accused of destabilizing the entire region," said Mark Edelman, assistant Africa administrator for the Agency for International Development, who led the U.S. delegation.

While many privately recognize Savimvi as a genuine black nationalist and some maintain ties to his movement, most believe that he has been tainted by his reliance on South African money and military assistance.