President Carter met with a critical Congressional Black Caucus yesterday and told its members he regards the unemployment rate among black teenagers as his "most important domestic issue right now," one of them said later.

Tht rate is now 40.4 per cent, and Carter was quoted as calling it "horrible." The President also was quoted as conceding another point black critics have made in recent weeks, that black joblessness has been caused in part by his administration's "lack of a comprehensive urban policy."

Carter's concessions had their intended effect. Rep. Parren Mitchell (D-Md.), the chairman of the caucus, said afterword that the members' mood was one of "cautious optimism."

The meeting "did not achieve our goal of getting a commitment to full employment . . . but there was a discernible movement on the part of the President in that direction," Mitchell told reporters.

Ten of the 16 members of the Caucus spent 45 minutes with the President - their first formal meeting with him since he was a candidate - and an additional 15 minutes with Vice President Mondale.

They along with a number of other black leaders, have been increasingly critical of Carter for not reducing unemployment faster. They hoped during yesterday's meeting to persuade him to endorse the Humphrey-hawkins full employment bill, which envisions a mixture of private and public service jobs to drive down overall unemployment to 3 per cent over four years.

Last week, afterthe Labor Department reported an overall black unemployment rate of 14.5 per cent, as against 7.1 per cent for the entire population, Carter asked for a report explaining why.

Labor Secretary Ray Marshall told him Tuesday there are three reasons ". . . other than discrimination . . . the concentration of black workers in central cities . . . their gap in levels of educational attainment . . . the overrepresentation of blacks in low-paying, less skilled jobs."

Marshall's memo also listedt wo other factors contributing to srarp increases in black unemployment in the past few months. One, it said, is ". . . the unbalanced nature of the current recovery where the largest employment gains have been in industries with a low concentration of black workers."

The other, the memo said, is that over the past 12 months many once-discouraged black workers returned to the job-seeking market, perhaps because they noticed the slow but steady drop in overall unemployment from December through May.

"This sudden surge in blacks seeking work is the major determinant of the recent increase in reported unemployment," Marshall's memo said.

The memo said that overthe past 10 years, the black population has grown much faster than black participation in the labor force, so that "many blacks without jobs were not reported as being unemployed . . ."

In the long run, it warned, ". . . the economy will be called upon to provide jobs for a black labor force that is growing at a much more rapid rate than the white labor force."

Yesterday's meeting was the latest turn in a controversy dating back to July, when Vernon Jordan, executive director of the National Urban League, publicly accused the President of ignoring the disadvantaged.

In response, Carter denounced critics for what he called "erroneous and demagogic statements." But black critics stepped up their attacks. Last week, White House sources reported a speedup in the administration's search for a comprehensive urban policy.

The differences with Carter over the Humphrey-Hawkins bill go back to campaign days, when blacks asked candidate Carter to endorse it fully and he declined for cost and other reasons.

The bill proposes public service jobs for those who can find no other work. Mitchell said Carter told the caucus the economy can absorb only so many such jobs, and that limit may have been reached by the 1.4 million that his new welfare plan envisions.