"LADIES AND GENTLEMEN," the secretary of housing and urban de- velopment begins, "I'm delighted to substitute for President Reagan . . ." What starts out as a few giggles picks up speed and turns into full-scale laughter.

Samuel Riley Pierce Jr. stands there impassively, a tall, distinguished-looking man waiting for several hundred men and women to quiet down. Behind him on the platform sits the hierarchy of Blacks in Government (BIG), an organization of federal, state and local employes that began a three-day leadership training conference at the Sheraton Washington Hotel yesterday.

Two days earlier neither Pierce nor BIG had expected to see each other. In May, invitations to address the conference had gone out to both President Reagan and Walter Mondale. By midweek, it was obvious neither would attend.

"I don't know how much lead time you have to have," Thomas O. Jenkins, BIG's national president, says later with a shrug.

Pierce, the administration's highest ranking black, whose lead time is apparently only two days, bears the laughter gracefully, then resumes the Reagan "economic pie" message.

"Today, our objective is to achieve equality in the economic sphere . . . We need the opportunity to climb to the top, build successful careers, own our own homes, share fairly in America's prosperity," says Pierce. "We're making great strides in that direction and this administration is helping."

A few among the well-dressed crowd slip out of their seats. Some spot friends and sit down beside them, others leave the ballroom. Determinedly, Pierce continues, his litany of statistics an effort to refute administration critics. One was his predecessor at the rostrum, Birmingham Mayor Richard Arrington.

"Jesse Jackson says there is something strange about blacks moving to the middle class," Arrington said. " 'They become afflicted with a disease called amnesia. They forget where they came from and they forget how they got there.' "

Entreated Arrington: "If we forget where we came from, we may well be going right back to where we came from . . . We should always remember that nobody will do more for us than we do ourselves."

Pierce turns to another matter, a flier that accuses the administration of "full-scale assault against black civil service employes." It is "simply not true . . . This administration will continue to work for full integration of minorities . . . We will not tolerate discrimination. You can take that from me and take it as the . . ."

And here laughter wells up again, this time drowning him out. Samuel Pierce, once known as "Silent Sam" among HUD's senior career civil servants and dubbed by U.S. News & World Report the "invisible man" of the Cabinet, strides off the platform, out the door and down a long corridor.

"This is a 99.9 percent Democratic group. I don't think they want to hear this. I give the facts and they don't want to hear the facts," he tells a reporter.

Is he insulted?

"Why should I be?" he says. "This is politics, sheer politics."

Sam Pierce denies that Reagan-Bush or the Republican Party has written off the blacks in Ronald Reagan's campaign for reelection. "Oh, no, they're fighting for every one they can get," says Pierce with every bit as much emphasis as he denies that he's not being used to full advantage.

"I certainly get a heck of a lot of places to go to talk all over the country. I've spoken for the Republican Party from Hawaii eastward. I've been to practically every state," he says.

Others, though, say Pierce is woefully underused. Except for a few media interviews in Dallas, the so-called "black presence" there was largely left to people like campaign aide Legree Daniels, White House assistant Wendell Gunn and former football star Rosey Grier. Dynamism is not one of Pierce's long suits.

"He's so bland that he's a turnoff," said a member of BIG yesterday.

Pierce can muster excitement in more intimate settings. At a barbecue in his honor at the now-legendary Southfork Ranch where the "Dallas" TV series was filmed, even aides were surprised by the crowd's enthusiastic reaction to his speech and his dancing. Tieless and wearing a western-cut suit, the wealthy New Yorker twirled his wife around the dance floor to everyone's delight.

That crowd of several hundred was an example of the blacks Pierce is referring to when he says more of them are benefiting from Reagan's "economic pie." Most weren't convention delegates, obviously, since there were only 73 black delegates among the 2,235 in Dallas.

"When I go out I find blacks showing good strength right here in Texas in the Republican Party, doing well. And these people are not wealthy but they do have economic security," Pierce said during an interview at the convention.

It's a point he likes to make about the Southwest -- an area he sees as "booming" -- whenever he has the chance.

"A lot of these folks have what I consider ordinary jobs but it's security, it's steady, they get good pay and have nice little homes. And I find a lot of these people moving toward the Republican Party," he said.

"I've never stopped being a Republican," said Pierce, the product of Republican parents and a Republican county on Long Island. When he was at Cornell, he considered switching, he said, but then he went off to World War II and never gave it another thought. When he came home, he discovered that some of his best friends were Republicans.

He went into law, became an assistant district attorney, an assistant U.S. attorney and later a judge. From there it was into government under the Eisenhower and Nixon administrations. When tapped by the Reagan administration, he was a senior partner in a Park Avenue law firm.

Pierce thinks blacks can be converted to Republicanism if a couple of things happen. One is giving them a better piece of that economic pie: he's quick to point out that since 1982, 1 million out of 6.7 million new jobs have gone to blacks. Another is making certain the administration doesn't "backslide" on civil rights.

"I emphasize that all the time because when Roosevelt came in, civil rights didn't mean very much to blacks. They really didn't have civil rights at that time. They gained them primarily in the '60s and they're not going to give them up. So if you want blacks, you can't backslide on civil rights," he said.

Yet a third "factor" is that blacks shouldn't "sell their souls" to either party, which is another place where Pierce comes in.

"You've got to have people like me constantly pointing out to blacks, 'Think about the fact that Democrats are beginning to take you for granted and not giving you the things you want,' " he said. "I think when they begin to get that sensitivity, they'll say, 'Yeah, that's right.' "

Meanwhile, Pierce's next scheduled campaign swing will be in San Antonio at a national Hispanic Chamber of Commerce meeting -- three weeks from now.