Unemployment in the District of Columbia jumped sharply to a rate of 9.6 percent in July but remained slightly below last summer's peak, according to a new D.C. Department of Labor report.

The report, circulated yesterday, blamed most of the increased joblesness on the seasonal entry of school-age youth into the local labor market. But there were no separate figures for youth unemployment, notably among blacks which is a nagging problem for local officials.

The jobless rate for the entire metropolitan area, including figures for the District of Columbia as well as the Maryland and Virginia suburbs, was 4.7 percent in July, well below the national average of 6.2 percent for that month.

The 9.6 percent rate for D.C. in July, which was seasonally adjusted, was up from 8.4 percent in June. It was slightly below 9.9 percent unemployment figure for July 1977.

According to the report, a total of 6,500 new workers entered the city's labor force in July, and only 3,400 of them got jobs. That meant that 3,100 were added to the jobless list. New workers do not qualify for unemployment compensation.

For the entire metropolitan area, a total of 13,200 entered the work force, and 12,800 got jobs.

Total employment in the metropolitan area in July was 1,518,200, with unemployment totaling 78,000, or 4.9 percent. In the District of Columbia alone, total employment was 305,600, with unemployment totaling 33,900, or 9.6 percent. For the suburbs alone, total employment was 1,211,600, with unemployment totaling 44,100 an unofficial rate of about 3.7 percent.

Despite the rather gloomy new figure, the report said the District of Columbia showed an improvement in balor market conditions over the past year. There were 6,800 more people working in July 1978, in the city than in the same month last year, and 600 fewer jobless.

There were few marked shifts in industry patterns over the year. Construction jobs increased by 500, probably reflecting a downtown building boom, and those in wholesale and retial trade rose by 1,400.

Government payrolls, the mainstay of the areawide economy, declined by 400 to a total of 286,000 in the city, compared with last year, but increased by 9,300 to 541,600 for the region as a whole. About one-third of the new jobs were with state and local governments.

However, the figures for government employment in July 1978 were 9,000 higher than in the previous month - mainly because of the D.C. government's summer jobs program for youth.

Earlier this year, before the influx of summer workers, similar monthly reports showed that the jobless rate had dropped 2 percent below comparable figures for 1977. That put unemployment almost at a level below the recession of the mid-1970s.

Statisfical breakdowns by age and race are made on an annual basis and were not included in these figures. When the figure for D.C. unemployment reached 9.9 percent in July 1977, that masked an official jobless rate for young black youth of 48.5 percent.

In its new report, the D.C. agency said the figures are not totally comparable with those contained in reports in previous years because of a new method of data collection introduced earlier this year by the U.S. Bureau of Labor Statistics.

There were few marked shifts in industry jobs increased by 500, probably reflecting a downtown building boom, and those in wholesale and retail trade rose by 1,400.

Government payrolls, the mainstay of the areawide economy, declined by 400 to a total of 286,000 in the city, compared with last year, but increased by 9,300 to 541,600 for the region as a whole. About one-third of the new jobs were with state and local governments.

However, the figures for government employment in July 1978 were 9,000 higher than in the previous month - mainly because of the D.C. government's summer jobs program for youth.