One of the best things we might do for the black "underclass," to which so much media attention has lately been given, is to let them in on our little secret: You can make it if you try.

I know that we think we haven't hidden that powerful light under a bushel, but in many ways, and for wonderfully altruistic reasons, that is just what we have done.

It's hard to find members of the black middle class who won't tell you that they experience some form of racism on a daily basis. It's harder yet to find black middle-class parents who don't keep pushing their children to work, to study, to compete -- drilling into them the notion that their exertions will pay off.

The problem comes when these same successful blacks speak in their role as advocates for the underclass. The message comes off quite differently: that their unsuccess is caused by racism and that an all-out attack on racism is the only way to fix it. The all-out attack clearly isn't occurring. Indeed, under the Reagan administration, quite the reverse is happening. And the underclass keeps growing. We take that as evidence that our race-based analysis is correct.

In some ways it is, but in other, in my view more serious ways, it isn't. The message we spread, to the detriment particularly of the young, is that members of the underclass exist solely as victims, not as rational people who can be persuaded to make the sort of choices we urge on our own children.

We may believe that our own prospects are artificially limited by an artificial ceiling predicated on race. But we also believe -- and act on the belief -- that our earnest efforts will produce enough of a pay-off to make the exertion worth while. That belief, as it turns out, is based on solid reality.

The Rand Corp. has just released a study revealing what successful blacks have known for a long time but that somehow has not entered our public debate. In the past 40 years, the proportion of blacks in the American middle class has tripled; the odds of a black man becoming wealthy have increased by tenfold. Black men, who earned only about 43 percent as much as their white counterparts in 1940, had, by 1980, achieved 73 percent of parity. Twenty-nine percent of black men have incomes above the average for white working men. (The statistics are based on detailed census data from 1940 through 1980.)

There's more. According to the researchers, James P. Smith of Rand and Finis R. Welch of Unicon Research Corp., the black gains are principally the result of improvements in their education and job skills.

But while all these wonderful things have been happening for the black middle class, the black underclass -- the bottom fifth -- has been "left out and left behind." The job market, theeneral economy, the shortcomings of the welfare system have all contributed to this state of affairs, and, I suspect, one more factor: our failure to drive home to the poor that they need not be passive victims of their fate.

We do say it to the poor people we know personally, but when we speak on behalf of the generalized poor, we often fail to stress this powerful fact. The sense that they have no effective control of their fate may be one of the reasons why so many poor people just let life happen to them, and why so many of them remain poor.

"Political rhetoric on the race issue," said the Rand report, "must eventually balance two compelling truths: America has made considerable strides in reducing black poverty; by the standards of a just society, black poverty remains at unacceptably high levels."

But neither the gains nor the poverty are things that just happened. The gains are the result of focused exertion and conscious investment, by individuals, of time, money and effort in improving their education and skills. It can happen for the poor, too, if they are made to believe that it can.

That lesson, which used to be imparted by social osmosis, when successful and unsuccessful blacks lived in the same neighborhoods, will have to be taught more directly now.

It won't solve all the problems of the black underclass -- there will still have to be government assistance for those who need it, organized efforts to improve the schools and protect their communities from crime, tax-supported programs to expand opportunity. But it would help enormously if the well-off would let the poor -- especially the children -- in on their vital secret: You really can make it if you try.