S. Africa's black boycott takes hold. Organizers of consumer boycott hope to hurt white business enough that leaders will pressure government for reform of apartheid policy

As international and United States economic pressure intensifies on South Africa to abandon apartheid, the nation's beleaguered white rulers face growing economic resistance at home in the form of a black-led consumer boycott. White owners of retail outlets are the immediate target of the boycott. But the ultimate target is the government. The objective is simple: to hurt white businessmen sufficiently to induce them to persuade the government to make political concessions.

The boycott started in mid-July in the troubled eastern Cape province. It spread to the western Cape and, more significantly to the densely populated area around Johannesburg after South African President Pieter W. Botha declared a state of emergency on July 21.

There is a direct relationship between the declaration of the state of emergency and the spread of the boycott. The boycott is one of the last forms of nonviolent resistance left to blacks, a spokesman for the anti-apartheid United Democratic Front (UDF) noted soon after the emergency came into force.

The growing spending power of the blacks, who have a population of 24 million, makes a consumer boycott a potentially powerful weapon. BOYCOTTBOYCOTT

According to Prof. Jill Natrass, of the University of Natal, in the past decade the blacks' share of total personal income has risen from 26 to 40 percent. Black consumer spending now accounts for nearly 50 percent of the annual growth in the consumer goods market.

The exercise of emergency powers by the police, their prohibition of mass meetings, imposition of curfews, and detention of black leaders, have increased the difficulties of organizing a boycott.

But there are clear signs that the boycott is starting to hurt white businessmen. One sign was a meeting of 80 representatives of the Association of the Chambers of Commerce in Johannesburg to discuss the boycott.

At the national level a joint statement by key commercial and industrial umbrella organizations showed the same concern: ``Organized business would seriously warn against the danger of the country entering a state of siege in response to the threat of local boycotts, trade union strikes, and stay-aways and international sanctions and disinvestment.''

The boycott is bringing a Draconian response from authorities. Police detained five black businessmen and closed their shops near Johannesburg invoking state of emergency regulations allowing closure of any business police think is promoting or prolonging a boycott -- in this case the businessmens' discussions with a local consumer boycott committee. The closure of shops is seen as a move to break the boycott of white shops by depriving black consumers of the opportunity of buying from black traders. The closure had all the hallmarks of a police bid to preempt the start of a boycott in surrounding areas.

In the eastern Cape the boycott helped bring a declaration from the the chamber of commerce in Port Elizabeth. It called for removal of legislation discriminating against blacks, inclusion of blacks in decision-making by the central government, and restoration of a common South African citizenship to all blacks.

Prof. Francis Wilson, of the University of Cape Town, commenting on the statement said, ``Clearly the business community is more in tune at the moment [than the government] with the wishes of the black South Africans. . . . The chambers of commerce are at last responding to the grievances that have been with the community for years and years.''