New figures show that, by the most basic of measures, the income of blacks in this country did not fall in the 1970s but rose, and in fact rose faster than did the income of whites.

The new figures look to per capita income rather than the more traditional measure of family income.

In 1970, according to Census figures, per capita income of blacks, expressed in constant 1980 dollars and including only cash, not Medicaid, food stamps and other in-kind benefits, was $3,966.

By 1980, that figure had risen to $4,804, or 21 percent.

For whites, per capita income in 1970 measured the same way was $7,118. It rose to $8,233 in 1980, or 16 percent.

These increases might appear even greater if in-kind benefits were counted, experts said.

Black-white income reports in the past generally have been in terms of "real" or inflation-adjusted median annual income per family.

By this standard, the one traditionally used by the Census Bureau, whites gained from 1970 to 1980, though only barely. Their median family income rose about $180 in constant 1980 dollars to $21,904. Blacks, on the other hand, clearly lost ground. Their median was down $650 to $12,674.

But Census experts say that these figures fail to take into account the shifts that occurred in the 1970s in family size. Black and white family size both declined, but black family size declined more.

And by the resulting bottom-line measure of real per capita income, while blacks remained far behind whites, they actually gained ground in the 1970s.

The figures leave no doubt that blacks still lag well behind whites in income. Some 32.5 percent of all blacks lived in what the government officially defined as poverty in 1980, as opposed to 10.2 percent of whites.

Hispanics, whose per capita income was $4,866 in 1980, had 25.7 percent in poverty. (The poverty line for a family of four in 1980 was $8,385.)

One reexamination of the traditional median family income figures on whites and blacks was done earlier this year by Gordon Green, assistant chief of the Census Bureau's population division, and Edward Welniak, also of the bureau.

In an unpublished paper they noted that black family size dropped from 4.26 persons in 1970 to 3.66 in 1980. The white figure also dropped, they noted, but less, from 3.52 persons to 3.2.

Reynolds Farley of the University of Michigan Population Studies Center is another expert who says the median family figures can mislead.

"Per capita income tells you more," he said in an interview. "There is every evidence of increases" in real purchasing power in the 1970s for both blacks and whites, although at a slower rate than in the 1960s.

The Green-Welniak paper strongly suggests that one major reason for the decline in black median family income is the rise in one-parent families among blacks.

In 1940 female-headed black families with children constituted 18 percent of all black families with children. In 1970 this figure had risen to 30 percent and today is about 42 percent. For whites, the figure today is 12.6 percent.

One-parent families tend to be low-income families, since they often have no or only one earner.

The Green-Welniak paper shows that among intact husband-wife families, especially those with two earners, black family income has actually gone up substantially and is now at a level of about four-fifths or more that of similar white families.