In a finding that appears to deepen the medical mystery surrounding the causes of high blood pressure, researchers have found that less-educated and poorer blacks with dark skin run a greater risk of the condition than do their lighter-skinned counterparts. However, among better-educated and wealthier blacks, the scientists uncovered no relation between skin color and high blood pressure.

The report, published in today's Journal of the American Medical Association, suggests that the social stresses of being poor and black in the United States play a much larger role than previously thought in explaining why black Americans are much more likely than whites to suffer from high blood pressure, or hypertension, and, as a result, suffer more heart attacks.

If the tendency toward high blood pressure were entirely or largely genetic, the researchers pointed out, they also would have seen a relationship between hypertension and skin color among wealthy blacks. As a rule, the lighter a black person's skin, the more non-black ancestors the person had and, therefore, the more likely that the person would have inherited any genes that might give whites their statistical edge.

One reason hypertension varies by skin color among poor blacks, the study hypothesizes, might be that darker-skinned blacks are more likely than poor blacks with lighter-skin to suffer from the psychological stresses that result from living in a race-conscious society.

But Johns Hopkins University epidemiologist Michael J. Klag, who led the research team, said the results are far from settling the debate over how genetics, racism and class contribute to the problems of high blood pressure among blacks.

"Minorities in this country have more health-care problems than Caucasians," Klag said. "This study suggests that for blood pressure, the genetic mechanism is less likely to explain that. But we have to work harder to fully explain this . . . We have only hypotheses."

The study looked at a random sample of 457 blacks drawn from three U.S. cities. The subjects were graded by their socio-economic status, according to occupational and educational level, and placed in one of four categories according to their skin shade. The reason skin color was measured is that the researchers wanted to test whether the genes associated with the African heritage of American blacks were in any way responsible for the higher than average levels of hypertension found among U.S. blacks.

The study showed a correlation between skin color and high blood pressure, however, only among the poorest blacks studied. For example, the darkest-skinned blacks in the lowest third of the socio-economic rankings had an average blood pressure of 148/99. A blood pressure of 140/90 is generally considered the upper limit of normal. The lightest-skinned blacks in the same disadvantaged socio-eocnomic category, however, had a blood pressure count of 133/84.

That relationship, however, did not extend to better-educated or wealthier blacks. The darkest-skinned blacks who had graduated from high school, for example, had a healthy and normal average blood pressure of 128/81.

The results were adjusted for a number of factors such as age, sex, weight and blood glucose level which could have skewed results.

Howard University geneticist Robert Murray Jr., who wrote an editorial accompanying the study, said the results did not rule out a role for genes. He said that there are a number of biological factors that put blacks at greater risk for hypertension than whites, such as the fact that blacks appear to be more sensitive than whites to salt in the diet, which can contribute to high blood pressure.

The study also raised the possibility that the high rates of hypertension among lower socio-economic classes might represent a combination of genes that makes blacks susceptible to high blood pressure and environmental circumstances that turn that into a problem.

However, a number of experts said the chief impact of the study will be to refocus attention on the effect that psychological stress and frustration caused by living in a race-conscious society and poverty have on the health of American blacks. "This study gives scientific support to something we have known for generations -- it's not easy being black in America," said Benjamin L. Hooks, executive director of the NAACP.