Democratic gubernatorial candidate Charles S. Robb overcame a strong personal appeal by his Republican opponent J. Marshall Coleman today to capture overwhelmingly the endorsement of Virginia's foremost black political organization.

Robb's strategists had said the endorsement by the Virginia Crusade for Voters was critical to the Democratic campaign. Robb has called the state's black voters "the critical margin of difference," and recent polls indicate they are responsible for Robb's lead over Coleman.

Today's endorsement should ensure Robb and his running mates of support by the vast majority of the state's black voters in the Nov. 3 election. Blacks comprise about13 percent of Virginia's registered voters, and most of the approxi-mately 9 percent of blacks who vote follow the Crusade's endorsements.

Robb won nearly all of the estimated 300 votes cast by members of the Crusade, which represents dozens of black civic groups in Richmond and throughout Southside and Tidewater Virginia. The vote came behind closed doors, after about three dozen black Republicans stormed out of the meeting charging it had been rigged by Crusade leaders to guarantee Robb's victory.

The vote also was a victory for Richmond Mayor Henry Marsh and state Sen. L. Douglas Wilder, two of Virginia's highest-ranking elected black officials, who had workedassiduously to overcome many blacks' doubts about Robb. His conservatism, his support for the Reagan administration's budget cuts, and his opposition to key provisions of the Voting Rights Act troubles many of them.

In a brief speech to the group today, Robb made no mention of those issues, talking instead in general terms. He told his audience, "We share common dreams and we can make those dreams a reality if we work together."

Coleman also appeared before the group, which had contributed to his upset victory four years ago in the state attorney general's race by endorsing him over a conservative Democrat with a segregationist past. Coleman reminded the group of that endorsement today, adding that he had kept his promises to maintain an "open-door policy" and to hire blacks to work on his staff.

Coleman then suggested that Robb's intense courtship of white conservatives was inconsistent with his appearance before the Crusade, declaring: "I don't have two speeches in this campaign."

"I don't have one speech for the Virginia Manufacturers Association and another for the Crusade," Coleman said. " . . . I don't have one speech for Roy Smith and Wat Abbitt and another for Curtis Harris and Doug Wilder." Smith and Abbitt are prominent white conservatives, while Harris is chairman of the state Southern Christian Leadership Conference.

Robb also was opposed by Jesse Jeffress, a Lynchburg accountant who is part of a write-in effort by Southside blacks disenchanted with both major party candidates.

"We have for so long been taken for granted by the Democratic Party and ignored by the Republican Party," Jeffress told the group.

Jeffress and his fellow write-in candidates yesterday accused the Robb campaign of buying the Crusade's endorsement by funneling "street money" to the group, a charge denied by the campaign and by Crusade chairman William S. Thornton. Jeffress today criticized black leaders for falling in step behind Robb, declaring, "A fool is a real fool when he starts to fool himself."

Although Coleman and Jeffress received some enthusiastic applause, it appeared that there were only a few dozen votes against Robb in the audience. This number was reduced to a handful after most of the black Republicans present bolted, charging Thornton with "purging" their ranks by saying they lacked proper credentials to vote.

"It was a stacked deck, and we're just not going to participate," contended John Handy of Hampton. But Thornton dismissed their complaint as sour grapes, and insisted, "They knew they were losing so, to save face, they left."

Robb, who has wooed black support in recent weeks by softening his stand on the Voting Rights Act, called the endorsement "obviously very important to me." In an interview after his speech, he rejected Coleman's charge that he was inconsistent in wooing black liberals as well as white conservatives, and explained that he is building "a broadly-based coalition . . . . Our tent is big enough for anybody who wants to work together."

While Coleman appeared today, his GOP running mates Nathan Miller and Wyatt Durrette did not, which led Wilder to charge that "the Republican Party has written off the black vote." Wilder said of Coleman, who won more than 30 percent of the black vote four years ago, "black votes elected him, but he opted for a new formula this time and he's made an horrendous mistake."