The pattern of an increasingly black city surrounded by increasingly white suburbs was broken dramatically in the Washington area during the 1970s, new census figures show.

At the start of the decade in the District of Columbia outnumbered those in the rest of the metropolitan area by 3 to 1.By 1980 nearly half of the area's blacks lived in the suburbs, as the city's black population declined during the decade by 89,483 while suburban blacks increased by 225,654.

Meanwhile, the total number of whites in the metropolitan area decreased for the first time, dropping by about 85,000.

In Washington, the racial balance remained almost unchanged at just over 70 percent black. In the suburbs blacks increased from 8 percent of the population in 1970 to 16.7 percent last year, a higher proportion that at any time since 1930, when most of the current suburbs were still farmland.

Overall, the population of the metropolitan area, which stretches from Charles County, Md., to Loudoun County, Va., grew by just 5.2 percent to 3.06 million during the 1970s, its slowest growth rate in the 20th century.

But within this overall stability, "there were major, major changes," said John McClain, chief of planning and forecasting for the Metropolitan Washington Council of Governments.

"Blacks became much more mobile than they used to be," McClain said. "Discrimination [in the housing market] declined, and the number of blacks with middle-class incomes really increased. . . . The old equation of blacks and urban just didn't stay constant."

According to census figures compiled in a new COG report, the increase in blacks since 1970 was greatest in Prince George's County -- 156,052 or 170 percent.

But the number of blacks more than doubled in three other large suburbs as well: Montgomery, Fairfax, and Prince William counties.

In Montgomery the black population grew by 29,205 or 135 percent, in Fairfax by 19,135 or 121 percent, and in Prince William (including Manassas and Manassas Park) by 7,683 or 130 percent.

The total number of blacks in Prince George's last year was 247,860, reaching about 37 percent of the county's population and accounting for 61 percent of the 404,814 blacks throughout the suburbs.

However, the percentage increase of blacks in Prince George's during the 1970s was less than the rate -- 196 percent -- it had reached there during the previous decade, while the percentage of blacks growth increased in every other suburb except Alexandria.

Data from school enrollment reports indicates that the surge of black population in Prince George's County may have slowed at the end of the 1970s. Since 1976, the year-to-year increase in black students attending Prince George's schools slowed from 3,692 to just 592 last fall. Meanwhile, the increases in most other, less heavily black, suburbs have steadily continued.

The new census data contains no information on the age of the population in the city and suburbs, but school enrollment reports indicate that much of the black increase in the suburbs is accounted for by families with children.

Last fall more black students were attending public schools in the Washington suburbs than in the city, a situation sharply different from 10 years ago when blacks in city schools outnumbered blacks in suburban schools by almost 2 1/2 to 1. The overall number of blacks living in the District -- 448,229 -- still exceeded those in the suburbs last year by 43,415 or 10.7 percent, but this gap was far less than in 1970, when the black population of the city was 358,552 more than the black population of the suburbs.

"The kind of housing that's offered by the suburbs is primarily family housing," said George W. Grier, a demographer who has studied area population trends for the Greater Washington Research Center. "And the blacks who are moving seem to be doing it for the same reasons as the whites who moved before them. They want play space for their children and schools that are regarded -- correctly or incorrectly -- as good."

McClain, of the Council of Governments said: "There seems to be more dispersal of blacks throughout the suburbs. They used to be concentrated in just a few areas. But look at a place like Fairfax. There are a significant number of blacks [34,994 in 1980] being assimilated into the county."

McClain noted that the black increase in the suburbs was far greater than the black loss in the District. He said this showed that not only are many black families moving from the city to the suburbs, but that many more apparently are settling in the suburbs when they first come to the Washington area.

Even though the number of blacks in the suburbs increased more than ever during the 1970s, the total black population for the area of 853,043 was up by just 136,171 -- less than two-thirds of the increase for the 1960s. McClain said this slowdown probably was caused by a decline in the traditional pattern of black migration out of the rural South as well as by a drop in births.

The area's total white population fell slightly during the decade, declining by 85,882 or 4 percent to 2.07 million. The drop included a decrease of 37,476 whites in the city, or 17.9 percent, which was much slower than the white loss there over the previous two decades, and a loss of 48,406 whites in the suburbs, or 2.5 percent, which was the first white loss ever in the Washington suburbs.

McClain said that the greatest part of the white loss occurred in Prince George's County, whose white population dropped by 170,049, or 30 percent. Smaller losses of whites also occurred in the close-in Virginia suburbs of Arlington and Alexandria. Whites continued to increase substantially in Fairfax, Loudoun, Prince William, and Charles, but this failed to offset the losses elsewhere as average household size dropped sharply because of fewer children, more divorces and more singles, young and elderly, living alone.

Also, McClain said, many families apparently moved to cheaper housing in counties just outside the metropolitan area.