U.N. Ambassador Andrew Young said yesterday he believes Sen. Edward M. Kennedy (D-Mass.) could capture a significant, perhaps decisive, share of the black vote if he challenges President Carter in 1980.

"I think there is no question about that," Young said on a television interview program.

But running for the presidency in 1980 doesn't seem to be on "Sen. Kennedy's agenda," said Young. Besides that, Kennedy "probably has the best record of support of President Carter of anybody in the Senate," said the usually free-speaking diplomat, who, this time, appeared to choose his words with caution.

Young's comments came the morning after separate appearances by Carter and Kennedy before nearly 8,000 persons at the eighth Congressional Black Caucus dinner, the top annual social-political event for black elected and appointed officials around the country.

Many diners viewed the two appearances as a preliminary contest for the black vote. And when it was over, they busily began issuing opinions on who had "won."

Carter spoke first at the Washington Hilton, one of the two hotels filled by the diners, and got three standing ovations with his vow to keep the controversial Young as UN ambassador "as long as I am president and he wants to stay there."

But Kennedy did equally well at the Shoreham Americana by taking some not-so-subtle shots at the president and by vowing to break the Senate deadlock on approval of the House-passed full employment bill by "attaching it to the tax-reduction bill on the Senate floor." The bill is the major piece of legislation being pushed by the Black Caucus.