A nationwide study has found that the writing ability of black school children improved over the past decade but that poor children overall have not closed the gap with their more fortunate peers in this crucial skill.

The improved performance of blacks is one of the few bright spots in a picture of an American school population whose writing abilities, on average, are about the same as they were in 1969.

As a whole, the study concluded, "There are some signs that the average quality of the writing [of 17-year-olds] is somewhat lower than it was," while 13-year-olds display a "significant decline" in descriptive writing and the overall quality of the work of 9-year-olds has improved.

"At all three ages, it appears that a considerable proportion of young people -- from 10 percent to 25 percent -- do not understand the nature and conventions of written language," according to the third survey of writing skills conducted by the National Assessment of Educational Progress.

The national assessment, which is financed by the federal government's National Institute of Education, is charged by Congress with being the nation's unbiased measurer of educational progress. In 1979, the national assessment reported that children had grown perceptibly worse in the previous five years in their ability to solve relatively simple mathematical problems. An evaluation of the reading ability of children will be released later.

In the writing study, assessment director Roy Forbes noted that there was material to support either optimistic or pessimistic views about the state of the country's educational system.

Most welcome, he said, was the news about black children. The study found that "black youngsters improved either absolutely or relatively on almost all writing tasks given to 13- and 17-year-olds and on one task given to 9-year-olds. In some cases this meant that they continued to perform below the national level, but not as far below as they had been in 1969 or 1970; in other cases this meant that they performed at the national level after once having been below it."

"I think we are beginning to see some payoff from our compensatory programs," Forbes said.

Charlotte K. Brooks, Washington representative of the National Council of Teachers of English, and said that "the evidence of growth in writing among blacks is significant and encouraging in the light of recent gloomy reports of black progress by Marian Wright Edelman of the Children's Defense Fund and Vernon Jordan of the Urban League."

Brooks, like Forbes, credited the federal programs of the last 15 years -- Title I, Headstart, and the Elementary and Secondary Education Act -- for reversing earlier trends that saw blacks falling further behind in writing skills.

The findings of the assessment were based on thousands of writing samples analyzed for coherence, grammer, fluency, persuasiveness and other factors.

The evaluation was done by giving children a variety of writing tasks: making up a story, persuading somebody, describing something, or supporting a point of view. One task was to make up a story about a stork; another was to persuade a landlord to allow tenants to keep puppies in their building. Papers were then evaluated on a numerical scale on the handling of each task.

Despite the good news about blacks, there was bad news about the writing skills of the total pupulation of disadvantaged children, which now includes a huge number of Hispanics. Although the writing of 17-year-old poor children in big cities improved, 13-year-olds in this category fell further behind and 9-year-olds' abilities remained the same relative to other children.

A. Graham Down, executive director of the Council for Basic Education, which favors rigorous, traditional curriculums, said it was "morally reprehensible" that "15 percent of our high school graduates are functional illiterates."

One of the findings described by Down, Brooks and Forbes as most disturbing was the steady decline in enjoyment of writing cited by children as they move through the school system, a fact that Brooks blamed on outmoded teaching methods that stress grading over practice and expression.

Although 80 percent of 13-year-olds wrote competent letters to a mail-order house and 75 percent of the 17-year-olds wrote competent narratives, "persuasiveness," which is viewed as indicating a child's grasp of the subject and thinking ability, declined in a number of samples between 1974 and 1979.

"We can be encouraged by these results but we haven't arrived at a place where we can be proud of what we're doing," said Forbes.