Fairfax County has made little progress in hiring blacks for top professional positions and as police officers and firefighters, new affirmative action statistics show.

The figures, released without comment by the county's Civil Service Commission, tend to support a decision by U.S. District Court Judge Albert V. Bryan Jr., who ruled March 31 there was a "pattern and practice of disparate treatment of blacks" in the county government's hiring. Bryan singled out three categories--professionals, technicians and protective services (primarily police, fire and jail) -- where the new figures show there has been little or no progress from 1978 through 1980.

"We're going to have to have a lot more staff interest in these areas," said Fairfax Supervior James M. Scott, who has been a consistent proponent of a more aggressive minority hiring program in the county. "We should do it not only because it is right, but because not to do so makes us legally vulnerable."

In ruling against Fairfax, Bryan ordered a monitoring of the county's employment practices. If he is not satisfied with the results of the county's efforts to hire more blacks he could set mandatory goals.

The county's personnel director, Cornelius J. O'Kane, has been widely praised by Scott and other officials for carrying a strong affirmative action message to various county agencies. The new statistics released by the Civil Service Commission do show that women are filling many more of the top positions in the county government, and that blacks are making significant gains in a few areas.

For example, women now constitute one-fifth of all those in the top paying administrator category and comprise 53 percent of those in the professional category, the second-highest paying group. Blacks made their largest strides in the paraprofessional category, jumping from 4 to 13 percent from 1978 to 1980 and in service-maintenance group, increasing from 8 to 29 percent.

But after three years of a purportedly strong affirmative action program, blacks still hold few jobs in key categories. For example, at the end of 1980, only 4 percent of the Fairfax police force was black. Fire and rescue did better, with about 9 percent being black by the end of the year.

Personnel director O'Kane said figures for the first half of 1981 "will show a tremendous spurt" in minority hiring by the police department. In the months of May and June alone, he said, 10 new officers will join the force.

o'Kane also said his office is preparing to send to the supervisors new voluntary affirmative action goals that, for many categories, will contain "substantially higher" targets for minority hiring.

The higher goals, he said, would reflect new guidelines that the federal government fought for in its successful discrimination suit against the county. The guidelines would be drawn up from the flow of applicants who seek employment with the Fairfax government.

If, for example, blacks constituted 20 percent of the applicants in one category, the agency responsible would be expected to hire about 20 percent blacks in order to be meeting its affirmative action goal.

The county has waged a long, bitter and costly fight against the federal government, over its minority hiring and recruiting practices. Board Chairman John F. Herrity, a Republican, took the county's case to Reagan administration officials, but was rebuffed.