PRESIDENT REAGAN'S explanation of his low standing with most of the black electorate, as offered in his last press conference, is that he just can't get through to it with the truth. He defined his difficulties as the product of an unwarranted "perception," denying that his initiatives have inflicted special damage on blacks and lauding his civil rights record and commitment alike. If he runs again, he said, responding to a widely voiced political question, he is not going to "cast any voters aside."

On that last point, surely he is serious. Pollster Lou Harris estimates that if blacks turn out in 1984 as they did in recent city elections in Chicago and Philadephia, and if they give the same 10 percent vote in the Republican column, then Mr. Reagan will need an 11-point margin from the rest of the electorate to win an overall majority of the popular vote. That's a big handicap, and it doesn't get any smaller if one studies the black vote potential in key states.

But what about that question of "perception"? Mr. Reagan projects great personal concern. But many blacks find that the facts send a different message. His claims of accomplishment are either so overstated ("We have increased our ability to help those truly at the lower earning end") or so tangential ("money that we have regained in wage disputes --we are setting a record in that") that his assertions often disturb listeners--and not just blacks.

Critics remember the administration's early efforts to weaken the Voting Rights Act. They know that the Justice Department's numerous enforcement actions ("21,000 inquiries into . . . suspected voting violations") stem mostly from the routine processing of redistricting initiatives accompanying the decennial census.

As to the effects of his budget and tax cuts, Mr. Reagan sounds as though he had been captured by glossy analyses from his budget office and elsewhere that portray the effects of social retrenchment as a minimal and relatively painless affair.

President Reagan insists he will defend any individual's constitutional rights "at point of bayonet, if necessary." No one doubts that he would. He fails, however, to acknowledge the diagnoses that many blacks make of their own condition. His is a voice full of compassion and good will, and of inadequate understanding and misdirected concern.

It is wrong and disappointing to hear his suggestion that blacks are somehow defective in their perception of what he is trying to do for them, or that they have somehow been deceived. Black feelings cannot be dismissed as ignorant or thoughtless. There are people, inside his administration as well as outside, who have been trying to introduce Mr. Reagan to a different and less congenial reality than the one in his head. He must entertain the possibility that it is he who misunderstands the consequences of his programs, not blacks who protest them.