The number of blacks, Asians, and Hispanics in Northern Virginia soared dramatically during the 1970s, the Census Bureau has reported, transforming what had been nearly all-white suburbs into areas that are racially mixed.

The change was greatest in Fairfax County, where the black population more than doubled in the decade, reaching 34,994 last year. The number of Asians in the county climbed to 22,725, an 11-fold increase, while the Hispanic population almost quadrupled to 19,535.

In Alexandria and Arlington, and Prince William counties there were similar though less substantial increases in all nonwhite groups, as the proportion of whites in Northern Virginia's population dropped substantially for the first time since the Civil War.

Overall, the number of blacks in Northern Virginia rose by 74 percent during the decade to 91,462, climbing from 5.6 percent of the population in 1970 to 8.2 percent in 1980.

"It's a remarkable break from a traditional pattern," said demographer George Grier, a partner in a local research firm. "Traditionally, the Virginia suburbs were considered by blacks to be off-limits, as places where they weren't welcome. But the fair housing laws changed that fact, and blacks have moved in. . . . Also, there's been an increasing availability of good-paying jobs to blacks so more can afford suburban housing."

"The Asians and Hispanics are in there, too," Grier added, "and that means a lot more color and diversity. We're not walling ourselves off as much as we did."

Demographers said it was the first time in a century that the number of blacks in the area grew faster than the number of whites, as the exodus of blacks from what had been rural areas bottomed out while substantial numbers of blacks began moving into suburban housing.

"The Potomac River doesn't seem to have been quite as large a psychological barrier [to blacks'] moving as it had been for generations," said Atlee Shideler, executive director of the Greater Washington Research Center.

Two weeks ago the Census Bureau reported that the black population of Washington fell by almost 17 percent during the 1970s -- the first time in more than 100 years that the District had lost blacks -- though the racial composition of the city remained stable from 1970 to 1980 at just over 70 percent black.

No data from 1980 census has yet been released on the population by race in the Maryland suburbs, though earlier state estimates showed the proportion of blacks rising sharply to 35 percent in Prince George's County and about 12 percent in Montgomery.

Overall, Fairfax County was the fastest growing of the close-in suburbs, with its population reaching 596,901 last year. Over the last 10 years, Fairfax had a 21 percent increase in whites, but this was far less than in previous decades. The nonwhite population rose from just 4.3 percent of all Fairfax residents in 1970 to 11.4 percent last year.

"Fairfax County certainly is diversifying," Shideler said.

Both Alexandria and Arlington recorded substantial drops of about 21 percent in the number of whites from 1970 to 1980. That was slightly more than the 18 percent decline over the same period in Washington's white population.

Meanwhile, Alexandria's black population rose by 47 percent over the decade, while the number of blacks in Arlington rose by 39 percent.

Even though blacks rose from 14 percent of Alexandria's population in 1970 to 22 percent last year, virtually all of the increase appears to have occurred before 1974, a period marked by a white exodus from the city's school system which undertook a major busing program for desegregation.

According to a survey by the Washington Center for Metropolitan Studies, Alexandria already was 22 percent black by late 1974. Since then the city's racial balance has been stable as whites have moved into renovated housing in Old Town while the average size of white families apparently has dropped elsewhere.

Alexandria has had substantial increases in both Asians and Hispanics, though the totals for both these groups are far less than in either Arlington or Fairfax. The Asian and other nonblack minority population of Arlington came to 8.1 percent last year, by far the largest proportion in this category for any part of Northern Virginia, though the actual numbers of these groups living in Fairfax County were much greater.

Census officials noted that there was a substantial increase in persons who simply marked "other" on the census' racial question. Many of these apparently were whites of Spanish origin who did not realize that there was a separate census question asking for their ethnic group. However, some Hispanics are black, and the census bureau is uncertain how many of them also marked "other."

The new count of Asians does not indicate to which nationality groups they belong. However, local officials believe most of the Asian newcomers are Vietnamese and Korean. Overall, the reported Asian population in Northern Virginia rose from 4,191 in 1970 to 35,716 last year. The Hispanic total rose from 14,515 to 37,550.

Of all the jurisdictions in Northern Virginia, Loudoun County, which is the most rural, was the only one with a negligible increase in blacks -- just 7 percent. Whites rose by 58 percent, reflecting new suburban development.