When Sharon Foston came from New York last summer to work as an insurance underwriter, she was one of five blacks working in her office. By the end of the year, three of them had been laid off in an economy move.

Foston, who still works at the firm, saw firsthand why women and minorities in particular fear the tightened job market. Coupled with the traditional policy of last-hired first-fired, it is eroding their recent advances in the workplace.

A recent congressional study of reductions in force in the federal government, for example, found that women and minorities were hit hardest. Between April and June of 1982, women suffered 46 percent of all RIF-related job actions although they make up only 29 percent of the federal workforce. Minority men and women, who comprise 19.9 percent of the workforce, bore 33.4 percent of the RIF-actions.

While such broad statistics are not available for private business, Anita Bellamy, director of the D.C. office of Human Rights, said it is clear that there, too, "blacks and women are becoming the primary victims of . . . economizing."

Many women and blacks fear that the economic constraints will foster a relaxation of affirmative action programs and a resurgence of discrimination.

Shelton's executive assistant, John L. Watkins, believes those fears are justified. The fact that 65 percent of unfair firing charges received by his agency in 1981 were accompanied by charges of prior employer discrimination is evidence, he said, that discrimination "is a vital reason for the disproportionate separations which black and female employes are experiencing."

"We are going through a period when companies are saying, 'We've done our thing.' And the economy is an easy excuse for that," said Sandy Jibrell, director of a private job consulting firm for youths.

"Can we honestly say that the more white males we see now is a product of economics or is it the growing unpopularity of affirmative action?" asked Denise Wilson-Taylor, a black lawyer.

Like their white counterparts, middle-class blacks are feeling the impact of lessened job mobility on their careers. But they say they feel particularly frustrated because it comes just as they were beginning to see the fruits of affirmative action programs.

"In our age group, the black professional women, everyone's hopes were high to get good jobs and we did," said Diana Montgomery, a federal merit-manager. "But with the present tenor of the country you realize you're frozen into a box. You realize there's no mobility."