Contrary to reports of deadlock and rancor, closed-door talks in Rhodesia aimed at moderate black government are showing conspicuous success - posing a challenge to official U.S.-British policies against any "internal settlement" presided over by Prime Minister Ian Smith.

Confidential official minutes reveal that, prior to holiday adjournment Dec. 21, the only major disagreement was over white minority representation in parliament, and even on the shape of a final compromise seemed to be emerging. Even more impressive is what the transcripts reveal about the tone of the dialogue between the Smith government, formerly dedicated to white supremacy, and black nationalists who have done time in Smith's Prisons.

Consider this Dec. 14 statement by Smith: "It is a fact that the [Rhodesian] civil-service system is white. It is also a fact that the education system is against the blacks . . . . I commit myself to seeing that no more blacks are denied these services. There are dramatic changes to take place. We don't want [armed] revolution because it is not good for us. Have faith."

"If I didn't have faith, I wouldn't be here," replied the Rev. Ndabaninghi Sithole, the black leader who returned to Rhodesia last summer after two years in exile.

Most revealing about the official minutes are not predictable about the official minutes are not predictable disagreements but the absence of rancor. Aside from mutual complaints by Smith and the blacks about "provocative" language, the only truly nasty encounter was between the two rival black leaders, Sithole and Bishop Muzorewa, over a side issue: which of them was leaking reports of the negotiating sessions.

If this congeniality produces an agreement, Washington and London will be on the spot. Disdaining the Salisbury talks, the United States and Britain insist that Joshue Nkomo - political leader of a Soviet-backed guerrilla force - must be present. Yet Nkomo has turned down pleas from Sithole to join the talks and is sustained in that intransigence by Lord Carver, British commissioner-designate for Rhodesia under the Anglo-American peace initiative. Currently in South Africa, Carver has attacked in advance any settlement at Salisbury.

The talks in Salisbury began Dec. 9 with Muzorewa proposing a British chairman to preside. The lack of strict racial alignment became clear when Smith, Sithole and tribal chiefs all opposed that suggestion.

Smith bowing to black insistence on 18-year-old voting, abandoned a 21-year voting age. The next disagreement came over Smith's proposal to "retain" Rhodesia's civil-service system while the black leaders wanted to "establish" one.

"The use of the word 'establish' will cause fear among whites because it connotes the doing away with what's here," Smith argued. He suggested a compromise with the word "maintaining "good standards."

Sithole replied, "I do not quarrel with 'maintain' in light of the explanation. Let's go on. I will compromise."

Quickly, the negotiators agreed on constitutional "safeguards" for the civil service, judiciary, pensions and other aspects of black majority government in the new Zimbabwe. They foundered on Smith's demand for a guaranteed one-third white membership in Perliament, needed to protect those safeguards.

Muzorewa on Dec. 13 accepted the one-third "blocking proposal but insisted that white legislators be elected by all voter, dominated 30 to 1 by blacks. "If white legislators are to be chosen by common roll," Smith retorted, "the blacks can elect puppets. It is a nonstarter."

"I appeal to you all to use less provocative language," Sithole admonished Smith. He later agreed with Smith that "if white representatives are to truly represent white interests, they must be chosen by whites." But Sithole wanted only a one-fifth white bloc in Parliament, insufficient for the one-third needed to block changes in the new constitution. He explained, "As soon as you make Parliament subject to blocking by a racial group you create strife."

Deadlock. Claiming he had heard "through my bush telegraph" of previous black agreement to nthe proposals now rejected. Smith declared Dec. 15, "Without a blocking third, I won't even start to sell anything to the whites . . . we will be laughed at." Sithole replied the "blocking third" would be derided by the outside world as a "sellout deal."

But in the Dec. 21 session preceding holiday adjournment, signs of hope appeared. Asserting that "we are not dogmatic on the method of election," Muzorewa indicated acceptance of a separate ballot for whites for the first few years but with whites limited to one-fifth, not one-third, of Parliament. Sithole seemed to suggest that, to begin with, a one-fifth vote could block constitutional changes.

That drew rough outlines for possible compromise as negotiations resumed Jan. 10. But what really points to compromise is the spirit expressed at the talks by a legal adviser to Muzorewa named Dumbuchena: "There is a difference between this conference and those before. For the first time, we have a measure of understanding and agreement." It is a difference that London and Washington so far refuse to appreciate.