It is hard to exaggreate the dismay of black Africa over what it regareds as the Reagan administration's tilt toward white South Africa.

Here, in the region dominated by the "frontline" nations of southern Africaa (Zambia, Zimbabwe, Tanzania, Mozambique, Botswana, Angola), the concern is especially acute, for their fate is inexorably involved with the future of the Pretoria government.

Ever since the election of Ronald Reagan, there have been hitherto suppressed fears in this part of the world that the new U.S. president would reverse or modify the pro-Africa policies of former president Carter, particularly his effort to liberate Southwest Africa (now Namibia) from long domination by South Africa.

These fears have now been confirmed by President Reagan's first statement on South African relations: "As long as there's a sincere and honest effort being made [to end apartheid], we should be trying to be helpful. Can we again take the other course? Can we abandon a country that has stood beside us in every war we've fought, a country that is essential to the free world, that has minerals?"

It was not surprising that these comments were promptly hailed in Johannesburg by Prime Minister P. W. Botha and other South African officials. Headlines in the Johannesburg press proclaimed: "US. Will Not Leave South Africa in the Lurch."

Washington, it was reported, has reversed the American policy of the past two decades. One report said, "Not only is it virtually the opposite of the adopted by the Carter government, but it is even more friendly than the policy of Richard Nixon."

The distress in black Africa was deepened by the fact that Reagan's remarks coincided with the start of a United Nations debate on South Africa's latest refusal to agree on a date to implement an independence plan for Namibia, where native liberation forces have been fighting South African troops for independence.

Reagan's statement is seen as the first step in a retreat from the plan, which was proposed in the first instance by the United States, along with Britain, France, West Germany and Canada. It called for a cease-fire supervised by U.N. peace-keeping forces, to be followed by elections and the drafting of a constitution, also under U.N. auspices.

South Africa accepted the proposal in principle nearly three years ago byt has always found an excuse for not putting it into effect. at a recent Geneva conference, it was accused of stalling until Reagan came to power.

On the heels of Reagan's new statement, the U.N. General Assembly, by a vote of 114 to 0, condemned South Africa for blocking a settlement and called for sanctions by the Security Council. The United States abstained, but there is little doubt that it will veto sanctions when the council acts.

The reaction in black Africa to Reagan's views was summed up by the Times of Zambia, which faithfully reflects official opinion. "America," it said, "cannot be a true friend of free Africa. Reagan's remarks have dashed any such hope. . . . Reagan has sold the conscience of the American people for investments in and profits from South Africa."

In friendly Kenya, where the United States is counting on bases for its Indian Ocean military operations. The Nation of Nairobi said, "It is obvious that Mr. Reagan has already come around to the ill-conceived view that stability in South Africa is essential to the promotion of Western interests in the region as a whole."

Noting Reagan's statement that South Africa was making a "sincere and honest effort" to stamp out apartheid, The Nation asked, "Whom is he kidding?"

The front-line nations are not in a position to strike back at the United States, except by turning to Russia for help, but the Times of Zambia is urging Nigeria to use the "oil weapon" against America.

Although few Americans are aware of it, Nigeria is the second-largest oil supplier to the United States -- so much so that last year the U.S. trade deficit with Nigeria was around $13 billion, the largest the United States has ever had with any trading partner. b

Moreover, Nigeria has been selling its oil to the United States at a price substantially below what it could get on the spot market. Nonetheless, when President Shagari of Nigeria visited the United States a few months ago, he pointedly said, " We cannot allow businesses to accrue profits from us which they will then use to support the oppression of our brothers in South Africa."

The Organization of African Unity, representing 51 of the continents's black nations, is also dedicated to the independence of Namibia. So the question seems to be: Should the United States risk alienating all of black Africa in order to ingratiate itself with an ever more isolated white South Africa?