Unemployment hit another post-World War II high last month as the rate moved to 9.5 percent from the 9.4 percent of April, but there were signs that the labor markets may be stabilizing, the government reported yesterday.

The work force expanded by more than 1 million persons, an unusually high figure for a single month that included the start of the annual surge of students seeking summer jobs.

Most of the million found jobs, and employment, which had generally been falling, shot up by 777,000.

Yet there were many who did not find work, and the number of unemployed, on a seasonally adjusted basis, rose by 242,000 to a total of 10,549,000, also a postwar record.

Black unemployment hit a new high, 18.7 percent. The comparable rate for whites was 8.5 percent. The rate for black adult males was 17 percent and for black teen-agers 49.8 percent, both also records. The rate for black adult women dropped slightly from 15.6 percent to 15.3 percent.

The new records notwithstanding, Janet L. Norwood, commissioner of labor statistics, saw evidence of some improvement. "After several months of steady deterioration, the labor market showed relatively little change from April to May," she told Congress' Joint Economic Committee.

Norwood noted that more people came into the labor force and found work than in other recent months, and that the hours of work edged up. Moreover, factory employment was relatively stable following sharp declines in previous months. Finally, an index covering some 200 nonfarm industries showed that 45.2 percent of them expanded their employment in May, the highest proportion since last September, she said.

Labor Secretary Raymond J. Donovan, citing several of the same points, said in a statement, "While we are concerned about the moderate rise in the unemployment rate, these signs give us hope that the recession may have at last reached its final stage."

However, a number of analysts said it is not clear whether the recession has hit bottom.

For instance, the extremely large increases in the labor force and the level of employment in May are statistically very unusual and could well be reversed in June, according to economist Alan Greenspan of Townsend-Greenspan & Co. Those figures are based on a nationwide sample survey of 60,000 households. A separate survey of actual payrolls at about 177,000 business establishments showed a small decline rather than an increase in employment.

In addition, Greenspan said the mix of hours worked among industries suggests the industrial production index for May probably will be down slightly. "The economy is essentially flat," he said. "Consumer goods markets are strengthening but capital goods markets are weakening. Forward orders are weak. There is nothing signaling an imminent upturn."

On the other hand, Greenspan also says the unemployment rate is at or very close to its likely peak for the year.

Even if a recovery does take hold in the second half of this year, as many economists expect, unemployment will decrease only slowly, Office of Management and Budget Director David A. Stockman cautioned. "I don't think there will be a precipitous decline. That's not the way the economy works," he said.

The continuing high level of unemployment may intensify congressional pressure for action to bail out ailing industries, such as homebuilding, in which joblessness is acute. President Reagan has said he would veto any such legislation.

Unemployment in some of the hardest-hit sectors of the economy and areas of the nation eased slightly in May, the Labor Department said. Unemployment among blue-collar workers fell from 13.7 percent to 13.5 percent. The rate for construction workers dropped from 19.4 percent to 18.8 percent. Michigan remained the state with the highest unemployment rate, although the seasonally adjusted rate fell there for the second consecutive month, from 16.1 percent in March to 15 percent in April and 14.3 percent in May. The rate also declined in Ohio, which has the second highest rate, from 12.4 percent to 11.7 percent.

In Illinois, which has the third highest figure, the rate rose from 10.4 percent to 11 percent. In California, it climbed to 9.5 percent from 9.3 percent.