Bernard Kalb, a veteran television correspondent who has covered diplomatic news for CBS and NBC, yesterday was appointed State Department spokesman to succeed John Hughes, who is resigning to return to journalism.

Kalb, 62, is the older half of the best known brother act in the news business. He and Marvin Kalb have been familiar faces to network TV audiences for years as foreign correspondents and, more recently, as a team covering the State Department diplomatic beat, first for CBS News and, since 1980, for NBC.

"There was an inevitability about the decision, and I am absolutely exhilarated," Kalb said yesterday. "This was nothing I sought to create or devise. It is something offered to me, an opportunity that came out of some marvelous blue."

The unexpected announcement at yesterday's daily State Department briefing provoked gasps of surprise and then an outburst of laughter and joking from other reporters who have worked with Kalb over the years. Underlying the merriment was their recollection of Bernard Kalb sitting in the end seat of the front row at the daily State Department briefing for the past decade and acting as the most aggressive questioner and persistent needler of a succession of department spokesmen.

In addition, Kalb's bombastic personality, jocular wit and fondness for garish shirt-and-tie combinations are in stark contrast to the retiring, publicity-shy personality of his new boss, Secretary of State George P. Shultz. It prompted several reporters to note that if Shultz is arguably the most colorless of secretaries of state, he at least will have the most colorful of press spokesmen.

In a statement, NBC News president Lawrence K. Grossman said that Kalb had "served NBC News well for many years as an outstanding correspondent" and that it will take "a lot of looking for us to replace him" in the position of culture and arts correspondent, a post Kalb assumed on Oct. 29 and one that grew out of his fondness for collecting porcelain antiques, he said. "It's a passion of mine."

While NBC News sources in Washington said Kalb was "truly and widely respected," one also acknowledged that "I don't think you could say he was overused" in recent years on network newscasts. With his move to the high-visibility State Department post, "At least now he's going to get on the air," the source said. Kalb said, however, that his leaving NBC News was no sign of displeasure with his assignment at the network or of dissatisfaction with Grossman.

Kalb's new job may be glamorous, but it doesn't pay very well. He will have to accept a cut in salary from the $155,000 sources say he makes at NBC News to the $69,900 a State Department spokesman said Kalb will make there. He will hold two titles concurrently: spokesman and assistant secretary of state for public affairs. The second title is "recognized in diplomatic circles" and carries some "protocol clout" with it, the spokesman said.

A substantial pay cut "is not something that crosses one's mind" when faced with a career move like this one, Kalb said. "To me, the overriding, irresistible element is the opportunity, and the honor, to serve the nation. I respond to that. Every opportunity I've had to do so over the years, I have characterized America as 'the unmatchable country,' and so I'm really very excited -- and speechless, I suppose -- at the opportunity to serve that country."

Asked if his appointment might be part of a Reagan administration ploy to improve relations with the press, Kalb said the thought had not occurred to him. "I am a professional newsman and I will handle this job with all the professionalism I can," he said.

Kalb recently returned from a trip to Poland where he visited the site of the Warsaw Ghetto. "I am the son of an immigrant," he said. "I took a hard look at the memorial to those who died in the Warsaw Ghetto. I know what this opportunity to be of service means."

State Department sources said that when Kalb returned two weeks ago from his trip to Poland and other East European countries, he telephoned Shultz to say goodbye in anticipation of his new NBC assignment. The sources said Shultz surprised him by saying that if he was looking for a new challenge, he should consider becoming the replacement for Hughes, who had announced his intention to leave.

Kalb, a native of New York City, began his journalistic career in 1946 with The New York Times and served as the Times correspondent in Southeast Asia from 1956 to 1961.

He joined CBS in 1962, serving successively as a correspondent in Southeast Asia, Paris bureau chief and Washington anchorman of the "CBS Morning News." In 1975, he joined his brother, Marvin, on the diplomatic beat. The brothers also are coauthors of two books: one on former secretary of state Henry A. Kissinger and the other a novel about the Vietnam war titled "The Last Ambassador."

As Kalb leaves journalism to enter government, Hughes leaves government to return to journalism. He plans to go back to Massachusetts, sources said, where he and his wife own a chain of small community newspapers. Hughes is a former editor of the Christian Science Monitor. He has been State Department spokesman since 1982.

Kalb was asked yesterday about an anecdote about him and his brother that has circulated for years. According to the tale, the Kalbs' mother phoned CBS News years ago when both her sons were working there and said to the person who answered the phone, "This is Marvin Kalb's mother; is Bernie around?"

"I've heard that story, and I often tell it myself," Kalb said yesterday. His brother had been at CBS News for many years before Bernard came there, Kalb said, and that may explain the way Mrs. Kalb phrased the question. "Of course, that's putting the best possible light on it," Kalb said. There is no sibling rivalry, only "sibling love," between the two of them, said Kalb, referring to his "dear, dear brother" as "magnificent" and "generous."