The Reagan administration, seeking to make a dramatic move to symbolize U.S. determination to reach out more openly to South Africa's dissident black leaders, is considering a black for ambassador to Pretoria, administration and congressional sources said yesterday.

The sources confirmed a Time magazine report that serious consideration is being given to nominating Robert Brown, a North Carolina public relations and management consultant, to succeed Herman Nickel, whose replacement was described by the sources as "imminent."

President Reagan has been forced by the escalating violence and repression in South Africa to order a review of U.S. policy there, but senior U.S. officials have made clear that the president intends to resist mounting congressional pressure to impose stringent economic sanctions against Pretoria's white minority government.

For this reason, the sources said, the impending departure of Nickel, who has been ambassador for four years, offers an opportunity to modify the administration's "constructive engagement" policy of seeking to effect change largely by using quiet persuasion with Pretoria. By giving a new, possibly black, ambassador a mandate for a bolder, more open approach to the black community, Reagan would be making a political gesture that could defuse much of the domestic criticism of his policies.

The sources said that while Brown appears to be the leading black candidate, the administration also is considering other prominent blacks. The sources also stressed that no firm decisions have been made about a black ambassador, and they said the administration could elect another approach such as appointing a special presidential envoy to deal with South Africa.

According to the sources, the pluses and minuses of different approaches are under vigorous debate within the administration. The aim, the sources said, is to demonstrate that while the administration opposes economic sanctions as counterproductive, it still supports political actions aimed at fostering dialogue between blacks and whites.

To this end, the sources said, the new U.S. ambassador -- whether white or black -- will be under instructions to expand greatly the U.S. embassy's outreach activities and begin closer public consultation with black South African leaders such as Winnie Mandela -- wife of Nelson Mandela, imprisoned leader of the outlawed African National Congress -- and Bishop Desmond Tutu, a Nobel peace laureate.

The sources said a draft of a speech emphasizing this aim has been prepared for possible delivery by Reagan prior to hearings on South Africa to be held by the Senate Foreign Relations Committee next week. However, the sources said, a decision has not been made about whether the administration can dramatize its case most effectively by having Reagan deliver the speech or by allowing Secretary of State George P. Shultz to outline the new policy approaches when he testifies before the Senate.

Brown, who served as a special assistant to President Richard M. Nixon, has close ties to American black leaders such as the Rev. Jesse L. Jackson and Atlanta Mayor Andrew Young. He has been consulted by the administration on ways of improving educational opportunities for South African blacks and, earlier this year, led a group of black educators to South Africa to help develop a teacher training program.

Questions were raised last night about an $860,000 Small Business Administration contract that Brown and a partner received in 1972, when Brown held a well-paying job in the White House, the Associated Press reported.

The contract was intended for disadvantaged minority firms, according to an aide to Sen. Lawton Chiles (D-Fla.), who learned of the contract during 1977 congressional hearings on SBA contracting practices.

Brown was not accused of any wrongdoing, said the aide, who spoke on condition of anonymity, but questions were raised about whether he had used his position to get preferential treatment. Congress later tightened the eligibility standards for minority contracts, AP reported.

Some administration officials, while acknowledging that a black ambassador could work to Reagan's advantage at home, say that such a move could be self-defeating in terms of diplomatic effects in South Africa, the administration and congressional sources said.

The sources said these officials argue that while President Pieter W. Botha's government probably would have to accept a black ambassador, it would consider his appointment a hostile act and would be unwilling to deal with him in any meaningful way. According to this argument, the net result is likely to be a further diminishing of Washington's already limited influence with South Africa's white establishment.

As a result, the sources said, one administration faction, reportedly centered in the State Department, believes that increased efforts to reach into South Africa's black community would be more effective if the campaign were headed by a career diplomat. State Department officials privately have said for weeks that Shultz's prime nominee is Richard N. Viets, a former ambassador to Tanzania and Jordan.

White House and State Department officials refused yesterday to comment publicly on the impending change of ambassador. They said only that Nickel, who is concluding a vacation in Europe, will participate fully in the policy review here.