For the chiefs of some 130 foreign missions it was an evening of debuts at Blair House last night.

At a reception hosted by Chief of Protocol Selwa Roosevelt, diplomats and their wives met her new Deputy Chief of Protocol William K. Sadleir and said farewell to his predecessor, newly appointed Deputy Assistant Secretary of State Thomas A. Nassif. At the door they were also met by the manager-to-be of Blair House, Sally Castleman.

As dean of the diplomatic corps, Soviet Ambassador Anatoliy Dobrynin was the ranking guest. His arrival set off a new round of speculation among his diplomatic colleagues about the possibility that he may be going back to Moscow for a high-level post there. Dobrynin, accompanied by his wife, Irina, did not linger, however.

Among the evening's diplomatic "debutantes" were several ambassadors-designate who are waiting to present credentials at the White House. Prominent among them were Zhang Wenjin of the People's Republic of China and his wife, Ying.

Zhang, who was last in Washington with Deputy Premier Deng Xiaoping in 1979, arrived here on March 4. In a brief chat last night with White House Deputy Chief of Staff Michael K. Deaver, Zhang said he is scheduled to present his credentials to President Reagan on April 7.

Asked by one guest if Chinese Premier Zhao Ziyang would be coming to Washington soon, Zhang replied in English, with the aid of an interpreter: "I'm waiting for instructions." And to a question about whether Reagan's travel plans will include a trip to Peking soon, he diplomatically suggested that it was for the president to answer.

Sadleir, who also was celebrating his 29th birthday last night, is former executive vice president of the Standard Energy Oil Co., with headquarters in Salt Lake City. He went to work a year ago in the White House scheduling office where his first assignment included coordinating American participation in the Versailles economic summit last June. He lived in Versailles from 1973 to 1975 where he worked for the Mormon church and became conversational in French.

Nassif, 41, a farm-labor lawyer in California before he came here two years ago, fills a position created for him at the State Department as liaison between the bureau of Near Eastern and South Asian affairs and members of Congress, government agencies and the public.

For a year he read everything he could get his hands on about the region, he said, then discussed the possibility of his new position with Secretary of State George P. Shultz, national security adviser William P. Clark, Deputy Secretary Kenneth Dam and Assistant Secretary Nicholas A. Veliotes.

"The State Department has a congressional relations bureau which basically deals with congressional staffs on the Hill. But you need someone who will deal with members," Nassif said. "I also had the opportunity, as deputy chief of protocol, to meet and travel with almost every head of state from that area of the world."

With Sadleir and Nassif in the receiving line were their wives, DeAnn Sadleir and Zinetta Nassif.

Sally Castleman, a Washington resident off and on since 1948 and most recently with the Nature Conservancy, will succeed Juliette Clagett McLennan as manager of Blair House when she takes over officially next Monday.