Prime Minister Pieter W. Botha agreed tonight to amend his proposals for a new constitutional framework in an attempt to win some black support for those plans.

Nevertheless, Botha's limited concession, which followed a three-hour meeting in Pretoria with conservative black leaders, is considered unlikely to make the proposals acceptable to much of South Africa's nonwhite population.

One of the most important leaders of the blacks' tribal homelands, Zulu Chief Gatsha Buthelezi, boycotted the meeting and made a blistering public attack on Botha and on what he called "half-baked constitutional arrangements."

Buthelezi, who heads the largest legal black organization in South Africa and has a considerable following among the 5 million Zulus, has asked Botha to postpone constitutional talks until some ground rules can be hammered out. The constitutional proposals Botha is now offering were drawn up by an all-white committee dominated by Botha's ruling National Party.

All nonwhite leaders have rejected those proposals, set out in May, because they provide for two separate advisory bodies, one for blacks and one for all other races. In addition, they are to be instructed to work out a constitutional order based on ethnic divisions and on geographically and politically distinct areas for blacks and whites.

A joint statement issued after today's meeting said the government is dropping the idea of a separate black council and will hold further discussions with the homeland leaders to work out an alternative structure. The homeland leaders indicated that they would be prepared to join a proposed president's council.

Even if blacks are allowed to join the president's council, however, the machinery is unlikely to gain credibility as long as its task is to work out a constitution based on ethnic divisions. Most nonwhite leaders favor a national convention at which all black leaders, including those in exile and in jail, could negotiate with the government on a constitution based on a common citizenship and power-sharing in a unified political structure.

This is the position of a delegation of black and white church leaders who met Botha yesterday in Pretoria to state their concern about the volatile situation in South Africa.

Botha told the church leaders that he rejected majority rule in unitary state and was working to form "some sort of confederation."

Today's announcement -- that the black council idea has been discarded -- is likely to bring Botha some criticism. His party's right wing is adamantly structure that could lead to power-sharing with the nonwhite majority.One of the reasons that blacks originally were not included in the president's council proposal was that the right wing opposed the idea.

One of the black homeland leaders who attended today's meeting, Cedric Phatudi of Lebowa, said of the government's concession: "I would not call it a victory, but progress toward the understanding of the aspirations of the black man."