The speeches and "thank you very muches" were slogging along at a Capitol Hill reception last night when suddenly: "All of these white people are liaisons and that's an insult," a voice cried. And then: "S---!"

The voice belonged to a tall woman in a flowing black pant suit, who turned out to be the Rev. Imagene Stewart of the Church of What's Happening Now. She's also a devout member of the National Black Republican Council, the organization that was holding the Hill reception to kick off a congressional conference today.

The reverend, everyone who knew her said, was just speaking her mind. She does this a lot. "In 1980, sweetie, you can't con people no more," she said a little later. Her main complaint was that the council's congressional liasion committee had too many whites on it. The council chairman, Jim Cummings, responded with: "We welcome her support and participation."

Interruptions aside, the official business of the reception centered on the council itself, an organization of 7,500 black Republicans. Its leadership is meeting with members of Congress today and is scheduled to hear testimony from other black interest groups and organizations. The goal: a platform to present at the Republican National Convention in Detroit next month.

Black Republicans are a rare but fervent breed, dedicated to corralling up more of themselves as well as railing against the Democrats in general and Jimmy Carter in particular. This, along with near psalms for Ronald Reagan (they endorsed him for president two months ago) set the tone at last night's reception.

"Carter reneged on every promise he ever made to blacks," said Cummings, a housing developer from Indianapolis who smokes narrow cigars. "The Democratic Party takes all blacks for granted. The Democratic Party feels it does not need to barter with or provide any services to blacks because whether there's services or not, blacks in greater numbers are going to continue to vote for it."

"Blacks are realizing that for us to survive politically, there has to be a strong two-party system," said Gene McFadden, director of the council's congressional committee and owner of three Big Fella's restaurants in Indianapolis. "The economic opportunities for blacks come from the free enterprise system."

And on Reagan, McFadden added: "We think Reagan did a good job in terms of the welfare system in California. We think he shaved it, customized the welfare system. Hey, I see people standing on the corners getting food stamps who need them as much as you and I."

About 150 people came to the reception, among them Reps. Millicent Fenwick (R-N.J.), John Myers (R-Ind.), Drew Lewis, the new deputy chairman of the Republican National Committee, and the usual assortment of young Hill staffers having dinner at the hors d'oeuvres table. During the speeches, four of them stood among the potted palms in Room 2168 of the Rayburn Building, drinking and eating cheese.