Unemployment among black young people is continuing to rise as joblessness among white youths declines, producing the highest summertime unemployment rate for young blacks - nearly 35 per cent - ever, recorded by the government.

This disclosure yesterday by Labor Secretary Ray Marshall came amid mounting charges by blacks and labor leaders that President Carter, who promised a reversal of such trends in his campaign, is ignoring the problems of blacks, cities and the poor.

Marshall said it is a "serious mistatement" to accuse the administration of inaction. But he conceded that government job-expansion efforts - including 2 million summer jobs for young people this summer - have widened the unemployment rate gap between young whites and young blacks.

According to Bureau of Labor Statistics figures comparing this July with last July, the overall unemployment rate for persons aged 16 to 21 dropped slightly, from 15.9 per cent to 15.3 per cent.

But all the improvement came for whites. While the unemployment rate for white young people dropped from 13.8 to 12.6 per cent, the rate for blacks increased from 30 to 34.8 per cent. The jobless rate for young blacks was 2.1 times as high as it was for young whites in 1976 and 2.8 times as high in 1977.

The unemployment rate for all ages and groups dropped from 7.8 to 6.9 per cent in the past year, according to BLS figures. The rate for all blacks increased, however, from 12.9 to 13.2 per cent, with young people suffering the most.

A review of statistics going back to 1967 indicates that, despite millions spent on federal programs ostensibly aimed at poverty and joblessness among minorities, young blacks are relatively worse off now than they were then.

In the past 10 years, unemployment among black youths, increased from 24.8 to 34.8 per cent, while the rate for whites increased only marginally, from 11.8 to 12.6 per cent.

Labor Department officials said the study did not touch on reasons for the mounting racial disparity in youth employment, but suggested that movement of jobs from predominantly black central cities to mostly white suburbs was a major factor, along with continuing racial discrimination and the effects of ghettos education and environment.

"The black kid isn't where the job is, and the white kid is," said one official. "When a middle-class white and a ghetto black shows up for the same job," said another, "guess who gets the job."

One official also suggested that federal jobs money is spread wide and thin, making the program politically palatable but less effective than it could be in reducing black inner-city unemployment.

Marshall coupled release of the midsummer youth job survey with announcement of three new Job Corps centers in Georgia California and Pennsylavania and the signing of an interagency agreement to operate the Youth Adult Conservation Corps - both part of a $1 billion youth employment bill signed recently by Carter.

He said these and other programs would have a "substantial effect" on youth unemployment, but declined to predict a reduction in the black jobless rate. "If it doesn't come down, it won't be because we didn't put a lot of people to work . . .," he said.

Marshall denied that the jobs figures are an argument for a youth differential in the minimum wage bill now before congress, saying it would have "catastrophic" effects on employment generally by substituting "white middle-class kids" for older workers, especially minorities,. He said it would not help black young people get jobs.

Proponents of setting a lower minimum wage for teenagers than for adults have argued that it would encourage the hiring of young people, including blacks.

But Marshall said the statistics are an argument for legislation to curb the flow into the country of illegal aliens, who, he said, complete most directly ith young blacks for jobs.