Washington, which 10 years ago had the highest proportion of blacks of any city over 50,000 in the country, fell to fifth place in the 1980 census, as the racial composition of the District held almost steady while the percentage of blacks in other cities rapidly increased.

Across the country, 15 cities with populations over 50,000 had black majorities in 1980, the census reported, up substantially from the seven in that category in 1970 and just two in 1960, Washington and Charleston, S.C. Among those with black majorities for the first time are Baltimore, 54.8 percent black, and Richmond, 51.3 percent.

The city with the highest proportion of blacks now is East St. Louis, Ill., on the east bank of the Mississippi River across from St. Louis; it is 95.6 percent black, compared with 69.1 percent in 1970.

In Washington, the proportion of blacks actually fell slightly during the decade, from 71.1 to 70.3 percent. For the first time, large numbers of blacks moved to the Washington suburbs while the exodus of whites from the city slowed substantially.

However, most other cities, particularly large ones, had a much faster out-migration of whites than in the previous decade. Even though the number of blacks in cities increased only moderately, and in some cases declined, the sharp drop in whites pushed the black percentage of the population up substantially.

In suburbs throughout the country, the black population soared by 70 percent, more than twice the rate it grew during the 1960s. Indeed, three of the cities that now have a higher proportion of blacks than Washington -- East St. Louis, East Orange, N.J., and Compton, Calif. -- actually are part of the older, inner suburban areas adjacent to much bigger cities. East Orange is next to Newark, Compton to Los Angeles.

Gary, Ind., the fourth city with a larger percentage of blacks than the District, is an old steel mill city that the Census Bureau classifies as the center of its own metropolitan area, though it is close to Chicago's suburbs.

According to the Census Bureau, the big city with the sharpest racial change dur-ing the 1970s was Detroit, although muchsmaller Inglewood, Calif., a middle-class suburb of Los Angeles, underwent an extraordinary transformation from 11 percent to 57 percent black.

In Detroit, the black population increased by 14.9 percent to 758,939, but the number of whites in the city dropped by more than half to just 413,730, propelling the black share of Detroit's population up from 43.7 percent in 1970 to 63.1 percent in 1980. Similar, though less drastic, trends in Atlanta pushed the proportion of blacks in that city up to 66.6 percent, compared with 51.3 percent a decade earlier.

Thomas Muller, a researcher for the Urban Institute, suggested that the list of white-minority cities might become substantially longer during the 1980s. In most of them, he said, the outward movement of whites will continue in search of better schools, less crime, and new jobs.

Many middle-income blacks will move out for the same reasons,Muller said, as they already have in Washington, but most of the poorest blacks will probably remain in the cities, which also are attracting large numbers of Hispanics.

In Chicago, blacks and Hispanics together already outnumberedwhites in the 1980 census, though there are still more whites in the city than either group alone.

According to the new census analysis, the 16.6 percent drop in Washington's black population during the 1970s was one of the steepest declines for any big city in the country, though it was exceeded by an 18.9 percent drop in St. Louis.

Muller said Washington's drop probably reflects the high proportion of blacks with middle-class incomes in this area, particularly from federal jobs, who can afford suburban housing. There also was a sharp reduction in housing discrimination in the suburbs because of federal laws, he said.

The slowdown in the white decline in the District -- which was 18 percent during the 1970s compared with 39 percent in the 1960s --probably occurred, Muller said, because most white families with school-age children had left Washington by 1970 while the city became increasingly attractive to childless professionals.

"I think we're seeing stabilization in those places that are relatively desirable, like Washington, San Francisco, and Manhattan, and where the economy is relatively strong," Muller said. "But I don't know about Detroit or Gary or places that aren't very desirable. There's no natural barrier at 70 percent black . When everybody who can afford to leaves a place, only the poor remain, and since blacks are disproportionately poor, there'll be more of them."

As cities gain a black majority, blacks often gain political control. This happened during the past decade in Atlanta, Detroit and Richmond. However, because the overall population of most big cities is declining, many cities are losing seats in state legislatures and Congress.

Besides the 15 cities that are majority black, there are 12 others where blacks exceed 45 percent of the population, including Memphis, Oakland and St. Louis.

Charleston, S.C., which was 51 percent black in 1960, dropped to 45.2 percent black in 1970 and was 46.5 percent black last year. Muller said Charleston has been a center of large-scale restoration of old houses, similar to the Georgetown and Capitol Hill areas of Washington, which has attracted whites while reducing the black population.