No part of the nation's structural unemployement problem seems so intractable as the joblessness of black teenagers. The Labor Department says that nationally, 31.5 percent were out of work last month. In some inner cities, the proportion is even higher.

The reasons-like those involving most of the structurally unemployed are complex. In Providence, R.I., Mile Van Leesten, director of a local job-training center, is upset because so many teenagers-black and white are coming out of public schools lacking even minimum skills.

In Van Leesten's view, a big part of the problem is "lack of motivation to study." So many jobs in Rhode Island, he notes, are "low-wage, low-skill, and deadend."

Manpower experts say the distinction is important. A white teenager may be perfectly happy taking a "deadend" job because he views it as only temporary-a way to kill time before fully entering the market for a serious career.

But to many black teenagers, whose parents are much more likely to have just those sorts of jobs as permanent work, these low-skill slots raise fears of a lifetime of deadend jobs. As such, they may not be as willing to take the first step on that road so blithely.

And, in any case, studies show that both whites and blacks are apt to quit such jobs after only a few weeks or months.

How can the government get a handle on this part of the structural umemployment problem?

No one seems to know, and some experts argue that it already is doing just about all it can or should.

Their argument runs this way:

True, black teenagers suffered from 36.3 percent jobless rate last year far higher than the 13.9 percent recorded for their white counterparts.

But while that rate for blacks may seem astronomical as a percentage numerically it represented only 381,000 persons-out of a teenage work force of 9.5 million.

Of that 381,000 at least 122,000 said they were looking for temporary jobs-not permanent openings.

About 150,000 were looking only for part-time work. And about three-fourths of these wanted after-school or weekend jobs only.

That long, dry string of statistics is not intended as an attempt to dismiss the plight of black teenagers who can't find work. For many, the problem is grim. And it does leave its mark, in terms of discouragement later about prospects for getting work.

Janet L. Norwood, acting commissioner of labor statistics, points out that while joblessness isn't a bread-and-butter issue for most teenagers including many black youths as well it is critical to some families where the teenagers is the only one who can work.

Still, the figures do serve to put the teenage unemployment problem into perspective-and point to a policy implication as well:

Manpower specialists say in a total labor force of 102.5 million-in which 96.6 million persons are employed it simply doesn't make sense to try to solve the black teenage unemployment problem with broad policies.

As such, some economists think the government shouldn't be spending as much time-or money-worrying about teenage joblessness.

Alan Fechter, a manpower specialist for the National Science Foundation, says the teenage problem is largely a function of demographics. When the rest of the postwar "baby bulge" reaches adulthood, he says teenage joblessness will lessen-just as it intensified earlier because of the bulge.

Moreever, according to University of Michigan economist George E. Johnson, between 30 percent and 40 percent of all the unemployed black teenagers in the U.S. already are benefitting from federal youth or manpower training programs.

In the absence of these programs, Johnson says, it's "quite likely that the unemployment rate of black teenagers would be much higher."

The teenage problem also is sensitive politicaly. To some politicians, the notion of large numbers of inner-city youths idle conjures up images of possible unrest and riots. President Carter and leaders of other major industrial nations have made reducing youth unemployment a major political plank.

Unfortunately, so far no one quite knows how to do it.