THE APRIL unemployment numbers provide modest but convincing evidence that the economy is on the comeback trail. But the numbers also warn that, for a lot of Americans, the job market offers scant cause for optimism about their prospects.

Since December, the economy has recaptured 650,000 of the payroll jobs it had lost in the recession and the unemployment rate has dropped by six-tenths of a percent. The gains in April, moreover, were widely spread throughout the hard-hit industrial sector. The labor force, which had dropped substantially in recent months, began to grow again, and work hours were up sharply.

The bad news is that total employment is still 1.3 million jobs below its level when the recession began in July 1981. Almost 2 million manufacturing jobs have been lost, with only part of that loss being made up by growth in the service sector. As the recovery continues, more jobs will come back, although the traditional pattern of a sharp rebound from a steep recession is not likely to be repeated.

With much uncertainty remaining about the durability of the recovery, employers are likely to proceed cautiously before rehiring. That will be good for profits and international competitiveness, but it will also mean that many formerly high-paid industrial workers will have to settle for a considerable reduction in their previous standard of living.

For blacks, the employment situation has not improved at all. Unemployment among blacks returned in April to the staggering 20.8 percent rate that it reached last December. Unemployment among black men of working age exceeded 20 percent. Only 16 percent of black teen-agers held jobs, compared with 42 percent of white teen-agers.

The administration has justified the harshness of its domestic budget cuts and the depths of the recent recession by the promise that the ensuing economic recovery would ultimately offer far greater benefits for all Americans. Thus far the benefits of recovery have not reached those Americans who have suffered the greatest losses.