Hundreds of blacks and women who applied for but didn't get jobs in the Fairfax County government are eligible for back pay if the county cannot prove they were not qualified or others were equally or better qualified, a federal judge has ruled.

U.S. District Court Judge Albert V. Bryan Jr. in Alexandria said back pay, including fringe benefits and 8 percent anmnual interest, could be awarded as far back as October 1975. However, Bryan said that any earnings from other jobs would be deducted from any back pay awarded to blacks or women.

Even though pay from other jobs would have to be excluded, a Justice Department official said the back pay potential "is enormous."

But Fairfax County Board of Supervisors Chairman John F. Herrity saw the ruling as a victory for the county.

"I don't think it [back pay awards] will amount to much," Herrity declared. "If there are some claimants who haven't had jobs elsewhere since they applied to the county, then that would indicate they probably weren't qualified in the first place."

Bryan, in spelling out his March 30 decision that found that Fairfax had engaged in a pattern of hiring and promotion discrimination, also ruled that the country has to offer jobs to eligible claimants "until all eligible claimants have had an opportunity . . . to accept such offers."

The claimants' pay for such jobs will be equal to what it would have been if they had been hired at their earlier dats of application.

Bryan appointed a special master, Thomas Rawles Jones, to carry out his order. Jones, will research all job openings that occurred from March 1972 in the categories where Bryan found discrimination.

In order to locate all blacks and female applicants who were not hired, the county must try to reach then by certified mail. The county must also publish notices in every daily newspaper in the Washington area and buy spot announcements on three radio stations, to be selected by the Justice Department, which filed the successful discrimination suit against the county. d