In the past two years, blacks have been moving out of the Northeast in substantial numbers and going back to the South or to the West, reversing a trend that has held for over a century, the Census Bureau reported yesterday.

"From 1960 to 1970 and in most decades since the Civil War, the Northeast had a net in-migration of blacks," the bureau said, "but these latest findings show a significant change of long-standing patterns of migration among blacks."

From 1975 to 1977, a net of 104,000 black moved out of the Northeast, the Census Bureau said. The bureau said many of those leaving Northeast appeared to be southern-born blacks who had come north long ago and now were returning to the South. A bureau official said the trend of black emigration from the Northeast apparently began in 1970-1975 but it was smaller. Now it is much larger and more pronounced.

"We don't really know the reason" blacks are leaving the Northeast, said one official, but many believe it is the same reason whites have been leaving the Northeast for a much longer period -- greater job opportunities in the South and in the Western part of the country.

In the North Central region, which like the Northeast had a massive influx of blacks over the past century, that migration pattern has ceased. The bureau said the number of blacks moving into the region in the past two years was about the same as the number moving out.

In the South, meanwhile, where there previously had been large net losses of blacks, emigration has leveled off and there has been no net loss. The western states had a net gain of blacks.

The changes in black population patterns appear to be bringing blacks closer to white patterns. For years, whites have been moving out of the Northeast and Midwest and moving south or west. Blacks now appear heading in the same direction.

Regionally, the South has by far the most black people, with 12.7 million age 2 or over. The north central states have 4.8 million, the Northeast 4 million and the West 2 million.

The census study also showed that the movement of all persons out of the nation's central cities to the suburbs or to nonmetropolitan areas continued over the past two years. The central cities lost 3.3 million persons age 2 years or older, the suburbs gained 2.7 million and the nonmetropolitan areas gained about 600,000.