A group of Prince William County black residents, charging the county discriminates against black job candidates and has an outdated affirmative action plan, has threatened to file a lawsuit if the county does not hire an affirmative action officer.

"We've been complaining for years," said Lorene Jackson, founder of Women Seeking Social Justice. "Many blacks have just given up and apply for jobs elsewhere. But then the county people . . . tell us, 'We don't get enough black applicants.' It's maddening."

Jackson and four other black residents met with county personnel director Cleil W. Fitzwater Jr. on July 1 but reported that the meeting was unproductive. Jackson said the group has sent a letter to the county Board of Supervisors, demanding that an affirmative action officer be hired.

Jackson said the county is well known in the black community for turning down qualified black job applicants in favor of white applicants.

Board Chairman Kathleen K. Seefeldt said she had not received the letter and would not comment on the charges. She said she has "no reason to think the affirmative action policy we adopted is not being followed."

According to an Equal Employment Opportunity Commission report that the county filed in 1982, about 8 percent of the county work force was black. The county employed 1,634 white workers, 136 blacks and 62 other minorities in June last year.

The affirmative action plan adopted by the county Board of Supervisors on May 6, 1980, called for a minimum of 5.3 percent black county workers by 1982. The plan was based on 1970 census figures that showed the county had a 5.3 percent black population. Jackson said about 11 percent of the county population is black today.

Jackson said Fitzwater told the group that met with him that the county should hire more blacks. She said he blamed low black representation on the board for not providing the personnel office with enough funding and department heads who do not hire qualified black applicants. Fitzwater declined comment yesterday.