Ever since Ronald Reagan's landslide reelection, with blacks voting overwhelmingly for the loser, we've been hearing talk of racial polarization. The explanations include the near- unanimous urgings of the civil rights leadership, visceral misgivings about conservatism in general and Reagan in particular and a difference in "perceptions" of blacks and whites.

It may be that the real explanation is what I call "the other shoe."

Read almost any report dealing with economic or quality-of-life indicators, and, no matter how upbeat the basic report might be, a few paragraphs into the account, they will drop the other shoe: "For blacks, on the other hand . . ."

It happened again last week, when the Labor Department came out with a report that had Reagan administration officials positively chortling. Civilian unemployment edged downward in February, as America's continuing economic vitality saw the creation of 300,000 new jobs.

Indeed, the number of working Americans reached an all-time high of 106.7 million. "We have seen a number of positive economic indicators in recent weeks," said the ecstatic Larry Speakes, presidential spokesman, "but none is more reassuring than a drop in unemployment. The economy is creating jobs in record numbers."

Then they dropped the other shoe. Joblessness for blacks increased by a statistically significant 1.4 percentage points. Since January, civilian employment for blacks fell by 174,000, most of the loss absorbed by adult black males.

That's not perception; that's reality.

And yet Americans have trouble understanding why blacks refuse to stand up and cheer the continuing good news. USA Today has been bubbling all week with its upbeat "Life Quality Index." The newspaper's polls suggest that 1984 came close to being the best of times. Four out of five Americans were happy with their pres their future prospects -- and with good reason. Their income was up; more than a third of them won job promotions; Reagan was in the White House and all was well with the world.

But like most such surveys, this one gives short shrift to "the other shoe." Low-income, jobless or marginally employed blacks (and those who presume to speak on their behalf) are keenly aware of it. And their pessimism is exacerbated by the fact that nobody in authority seems to be paying it much attention.

The Labor Department's Bureau of Labor Statistics, which issued last Friday's report, didn't even attempt an explanation, except to say that black employment tends to be volatile.

For blacks, on the other hand, explanation is crucial. It is not merely statistically intriguing but downright frightening to note that when things go badly for the general economy they go badly for blacks, but when things improve dramatically for the general economy they still go badly for blacks.

And because nobody in authority tells us why, we are left to argue among ourselves as to the most likely explanation. Is it racism? Is it the fact that jobs are increasing fastest in those parts of the country (or in those industries) where blacks are least likely to be? Does it have something to do with education or job qualification? With overreliance on government jobs or transfer payments or the decline of affirmative action?

Not only does no one seem to know, but the government seems uninterested in finding out. This apparent lack of interest, probably more than all the other explanations combined, may account for the political polarization that has the commentators so fascinated.

If white America believes, on the basis of its own quality-of-life indicators, that the Reagan approach is working, small wonder that it should have voted so overwhelmingly to return him to office. If black America knows that the rising economic tide is leaving its boat stuck at the bottom, is there any mystery why it should have voted overwhelmingly for the other guy?

And if Ronald Reagan really cares -- either as the nation's chief Republican or as the president of all the people -- shouldn't he be trying to find out what the hell is going on?