S. Africa shunting blacks to Swaziland?

South Africa's mapmakers have taken eraser in hand once again.

The government has announced a plan to redraw South Africa's borders in a way that hands over 800,000 blacks and 1.8 million acres of land to the tiny nation of Swaziland. Pretoria says the move would reunite the Swazi people.

Critics, including black leaders of two South African territories whose people would become Swazi citizens, see it as another step in the long-range policy to expel blacks from white-ruled South Africa.

The border adjustment proposal adds an international dimension to South Africa's much-criticized ''homelands'' policy, which separates blacks from whites by designating tribal areas as black states. (South Africa has designated four of these homelands as ''independent'' nations, but no other country has recognized this independent status.)

The twist in the latest proposal is that a recognized foreign state is involved. Under the plan, Swaziland would absorb the KaNgwane homeland and the northern part of KwaZulu, which includes Kosi Bay on the Indian Ocean.

KaNgwane and KwaZulu had refused to accept independence from the South African government. But the effect of this latest plan would be the same on South African Swazis as under independent homeland status: They will lose their South African citizenship with no choice in the matter.

Analysts here question why Swaziland has agreed to the plan. As a member of the Organization of African Unity, it is expected to come in for sharp criticism from that body for, in effect, aiding South Africa's homelands policy, which the OAU condemns.

Further, the proposal could very well prove destabilizing for the Swaziland monarchy of King Sobhuza II. The Swaziland population would more than double (it is now 500,000), adding sharp new financial demands on the small kingdom's government.

Also, the South Africans who would become Swazis seem resistant to the idea. Many of South African Swazis have been ''detribalized,'' according to Prof. Gavin Maasdorp of the University of Natal, and the proposal would force them to adjust back to a more traditional form of rule.

Although the South African government insists most Swazis in South Africa favor the proposal, what evidence there is seems to suggest otherwise. The KaNgwane legislative assembly voted overwhelmingly against incorporation before the South African government dissolved the legislative body. A petition of Swazi chiefs in South Africa also protested the plan. KwaZulu Chief Gatsha Buthelezi has strongly objected to the proposal as well.

On the plus side for Swaziland will be the new land, some coal reserves in KaNgwane, and access to the Indian Ocean. Swaziland has long claimed to be the rightful owner of much of the land in question.

Access to the sea could be beneficial economically if the currently landlocked kingdom were financially able to develop a port and the necessary transport links. The sea also has some ceremonial importance. Each year the Swazis collect water from the Indian Ocean for their Incwala Festival - a celebration of the ''first fruits'' of the harvest.

Swaziland has denied it is giving South Africa anything in exchange for the land and its inhabitants. But speculation is strong here that secret security agreements may be involved. Swaziland, situated on the border of Mozambique, is often the transit point for saboteurs entering South Africa.

Speculation also centers on Swaziland being maneuvered, knowingly or not, into a position to join in Pretoria's plan for a ''constellation of states'' in southern Africa.

The constellation concept, calling for regional cooperation among South Africa, the homelands, and other black states, has been rejected by black Africa.