The 1970s were a decade of overall racial stability in Washington, the U.S. Census Bureau reported yesterday. Blacks made up just over 70 percent of the 637,651 city residents counted in 1980 -- almost the same share they had a decade earlier.

The District's total population fell by 16 percent during the 1970s, the census bureau said, and the parallel decline of both blacks and whites was sharply different from the pattern of the two previous decades, when the black population almost doubled and the number of whites dropped by more than half.

According to earlier city government estimates, the District's white population increased slightly for several years during the late 1970s, but the turn-around was far from enough to offset the substantial decline in whites over the first part of the decade.

"It looks like the racial composition is stable," said Albert Mindlin, the District's chief statistician. "Washington's certainly not turning into a white city. . . ."

Although no detailed information is available yet on specific neighborhoods, whites have visibly increased in some inner-city areas, such as Adams-Morgan, Capitol Hill, and Shaw. This phenomenon has caused some poor blacks in those neighborhoods to move elsewhere, and has created fears among some prominent blacks that whites might "take over" political control.

"Clearly, the trend toward black suburbanization has accelerated," said Atlee Shidler, executive vice president of the Greater Washington Research Center, "and the rate of loss of whites is way down . . . But the blacks going to the suburbs seem to be people with better-paying jobs who want better housing."

Mindlin, the city statistician, said "The so-called 'gentrification' [renovation of old housing in poor neighborhoods] has been a relatively minor phenomenon up to now. It's likely to increase, but I don't think it's likely to mean a major change throughout the city."

The new census report, based on the official count of last April 1, shows that Washington's black population fell by 118,859 between 1970 and 1980. The city's 1980 black total was 448,229 -- down 16.6 percent from a decade earlier.

Census officials said this was the first census count since before the Civil War to show a decline in the District's black population. The change was sharply different from the 31 percent increase in blacks here during the 1960s and the 47 percent increase in the previous 10 years when, in large parts of Washington, blacks replaced whites who had moved to the suburbs.

During the 1970s the District's white population continued to fall, the census said, from 209,272 in 1970 to 171,796 last year. But the decline of 37,476, or 17.9 percent, was far less than in the 1960s when Washington lost 135,991, or 39 percent of its white population.

According to census bureau estimates, which are not based on actual counts, the decline in the white population was slower in the second half of the 1970s than it had been before 1975. Meanwhile, the black population loss accelerated sharply after 1975.

Overall, the proportion of blacks in Washington's population dropped from 71.1 percent in 1970 to 70.3 percent in 1980. The proportion of whites also dropped slightly -- from 27.7 percent to 26.9 percent.

The difference is accounted for by a substantial increase in persons who identified themselves as belonging to other races, though the numbers in these categories are still relatively small.

The city's Asian population rose from 5,372 to 6,635, the census bureau said, while American Indians and Eskimos increased by 75 to 1,031.

The most striking increase was in persons who simply marked "other" on the census' racial question. This group rose in Washington from 3,198 in 1970 to 9,960 last year. The same major increase in the "other" racial category occurred throughout the country, the census bureau reported several days ago, probably because of ambiguities on the census form.

Most of the 6.8 million persons who checked "other" nationwide 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 the Hispanics who did not mark white or black actualy belong to each race. Officials think the net effect of the problem was to artificially reduce the white group, though in Washington the impact on the racial count appears slight.

The District' Spanish-origin population was 17,652 in 1980, up from 15,108 the decade earlier.

Earlier analyses of the District's population change, based most recently on a 1977 housing survey, indicated that the main factors in the sharp loss of blacks were the movement of middle-income black families to the suburbs, particularly Prince George's County, and the substantial reduction in average household size because of a drop in births and an increase in divorce and single-parent homes.

Even though massive numbers of white families left the city in the 20 years before 1970, Mindlin suggested that those remaining were affected by the national trend toward fewer children and smaller household size.