''President Bush did not know I had been here,'' she said, recalling his telephone call to her in Seattle on Feb. 28 to ask her to take the job. ''I said yes so loudly and quickly that Charlie asked me what it was I had agreed to do,'' she said, referring to her husband of 39 years, a California businessman. ''It means a great deal to me.'' Delegate to United Nations

Mrs. Black's diplomatic career began in 1969 when President Richard M. Nixon made her a United States delegate to the United Nations. President Gerald R. Ford made her Ambassador to Ghana in 1974, and later the first woman to hold the position of White House Chief of Protocol. That ended when a Democrat, Jimmy Carter, took over the Presidency in 1977.

''I remember telling President Carter on his first night, when I was escorting people around, that I was interested in continuing public service and that politics didn't matter - but it does, doesn't it?'' she said with a laugh.

Mrs. Black brings many surprisingly apposite qualities to the new job. She has, as she described in her 1988 autobiography ''Child Star,'' published by McGraw-Hill, an actor's almost photographic memory, the instinctive feel for the ways of professional diplomats (saying today is ''31 August'' instead of ''August 31,'' for example), and the skills of personal diplomacy that put her young Marine guards, who may have heard about her movie career from their grandmothers, if at all, as much at ease as Mr. Husak is with her.

What she says she wants to do now is improve United States relations with Czechoslovakia and urge the changes here that are necessary to bring about that improvement - an end to restrictions of human rights, the freedom of speech and assembly. One-Way Ticket to Prague

She said she recognized that it would not be easy. Mr. Husak and his colleagues are in power because they clamped down on those rights at Russian insistence in 1968, long before Mikhail S. Gorbachov had allowed the winds of change to blow from Moscow and through Eastern Europe.

''I was in Vienna in August 1968 for a meeting of the International Federation of Multiple Sclerosis Societies, of which I was co-founder, and we wanted a 20th country to join,'' she said. ''They asked for a volunteer to go to Prague to get Czechoslovakia to do it, and my hand always goes up first.''