Shirley Henderson, a 26-year-old mother of two, thought about voting Tuesday. She was still thinking about it when the poll two blocks away closed.

"I was going to go at first, but then I thought, 'Reagan's going to win anyway,' " she said from her kitchen in a public housing project.

Three miles away, Carole Crandall, 36, fully intended to vote, and did -- for a Republican senator and a representative.

It was people like Henderson and Crandall, both black, who foiled Democrats' expectations that blacks would provide a strong Democratic voting bloc on Tuesday.

Thousands of blacks in the state entered their names on the voting rolls this year, partly because black activists fought for more registration sites in black communities and partly because of the influence of the Rev. Jesse Jackson, who turned out more participants than any other candidate in the state caucuses in May.

But a Washington Post survey of 21 predominantly black precincts that were targeted to register voters in Newport News, Richmond, Norfolk and Portsmouth showed that many like Henderson stayed home. Less than 75 percent of blacks who registered in those precincts voted -- compared to an overall turnout of more than 78 percent of registered voters in those cities.

And a surprising number of those who did vote chose GOP candidates. Republican Sen. John W. Warner, for instance, racked up an average of 33.5 percent of the vote in nine heavily black precincts in Richmond.

In Newport News, Republican Rep. Herbert Bateman, who voted against the federal holiday to honor Martin Luther King Jr., won as much as one-fifth of the vote in some largely black precincts where his Democratic opponent John McGlennon was hoping for overwhelming margins.

"There was such a strong Republican tide running, it even spilled over into the black district," McGlennon said.

Crandall was one of those caught up in the tide, to her own surprise. Though she cast her vote against Reagan, she pulled the lever for Warner and Bateman because, she said, "too much liberalism, I think, is bad for the country." She paused for a moment and added, "I can't believe I'm really saying this, but this is how I feel."

Cynthia Downs, president of the Newport News chapter of the NAACP and a Jackson organizer, orchestrated the registration drives for new black voters in that city. To her, voters like Crandall are a sign that "the black vote is tending to be more and more independent. You can't just assume anymore they're going to vote Democratic."

Warner didn't. He sent out letters to black voters citing his support for a King holiday and employed an aggressive black organizer. Precincts that gave President Reagan only 5 to 7 percent of the vote gave Warner 25 to 35 percent on Tuesday.

Downs and other Democrats believe Democratic candidates didn't make enough effort this year to persuade some voters to go to the polls. They said Walter Mondale wrote off the state and Edythe C. Harrison's race against Warner showed little life. Even the $35,000 to $50,000 that the state Democratic Party spent on get-out-the-vote efforts -- more than they'd ever spent, didn't begin to fill the gap, said Bernard Craighead, political director of the state Democratic Party.

McGlennon used the money he got from the party to hire 66 young black teen-agers, who worked as "flushers," all Tuesday, hanging reminders on doorknobs and knocking on doors in black neighborhoods in Newport News. In addition, volunteers worked six telephone lines at the local steelworkers union for about a week before the election.

But that was only a fraction of the effort Sen. Bobby Scott (D-Newport News) put out to win his seat in the General Assembly last year. Linwood DeBrew, who worked in both McGlennon and Scott's campaign, said Scott, who is black, sent out 210 "flushers" and had numerous phone banks. "We worried them out to the polls," said DeBrew.

McGlennon said his effort "wasn't enough," but added that even if blacks had turned out in huge numbers, he still would have lost, since black votes alone would not have prevailed without the necessary white support in the district.

Traditionally, black turnout has lagged a few percentage points behind white turnout. Nevertheless, blacks in Virginia have so increased their registration that even with a relatively low turnout this year, they still slightly increased the percentage of those who voted in the precincts surveyed compared to 1980.

The Voter Education Project in Atlanta estimates that 97,000 blacks have registered in Virginia since 1982, an increase of 29.4 percent.

While Larry Sabato, a political science professor at the University of Virginia, said he believes that estimate may be inflated, he said that blacks have cut the gap in half between black registration and white registration in the state since 1980 -- from 10 percentage points to 5 at the most.

At least in registration, "Blacks have caught up a lot," Sabato said.