The editor of the largest black South African newspaper yesterday characterized U.N. Ambassador Andrew Young as "a refreshing tonic" to the blacks there.

Percy Qoboza, editor of World, who is on a visit to the United States, said that the outspoken black U.S. ambassador to the United Nations "should not have to aplogize for what he said about the South African government being illegitimate. He is saying things the United States should have said a long time ago."

Told yesterday about Botha's warning that Young would be barred from South Africa if he planned to meet with opposition black groups without the government's permission, Qoboza said, "I'm happy, because you [Americans] will realize what kind of people you're dealing with."

Qoboza, who once had a Nieman fellowship at Harvard University, said he did not trust Botha, a former ambassador to Washington who is regarded as a moderate within the ruling Nationalist Party. Qoboz said pressures on Botha within his party will force him to move to the right.

Qoboza was interviewed with Ton Vosloo, white editor of Beeld, an influential Afrikaner newspaper that generally supports the South African government.

"What would you do if you were me?" Qoboza demanded of Vosloo.

"I would share the same aims and ideals as you," Vosloo replied.

Vosloo nevertheless defended the South African government's plans to revise the constitution, which he said would allow for participation of mixed race and Indian citizens in the national government, now restricted to representatives of the country's 4 million whites.

Vosloo called the proposed changes a "radical" step for his government but he admitted that these reforms would not go to the "nitty-gritty" of South Africa's problem because the country's 10 million urban blacks would still be excluded from the political process.

While these constitutional amendments might be a big step for the Afrikaner-dominated government, Qoboza said they would mean nothing to the black people.

"I get depressed when I think that the dreams and visions of the white people keep falling down," Qoboza said. "They will take five years to study the issues, another seven to make recommendations and another 10 to write up a white paper which will reject most of the recommendations. What do you expect us to do all that time, twiddle our thumbs?"