Blacks continue to be dramatically under-represented in the Washington-area construction trades despite a decade of effort to increase their numbers, witnesses at a D.C. government hearing agreed yesterday, although there was no agreement on who is to blame or what should be done.

Anita B. Shelton, director of the city's Office of Human Rights, which called the day-long hearing, said in an opening statement that despite years of attention to the problem, only 10 percent of journeyman construction workers in the District are black although the city is 70 percent black.

"I believe there is a conspiracy" against blacks in the construction field, said black contractor Alfred Harrell, who seemed to sum up the frustration of many of the witnesses who testified before Shelton and a group of city officials.

Officials which operate apprenticeship programs leading to full journeyman status in the building trades like plumbing or metalwork, said minority representation in the skilled trades has nearly doubled in the last decade.

But the progress falls short of the goals of the highly touted 1970 "Washington Plan," a federally imposed affirmative action program designed to increase black representation in 11 key skilled trades in Washington.

Dave Robinson, president of the Washington Building and Construction Trades Council, said that in 1970 minorities made up only 23 percent of skilled tradesmen in the Washington metropolitan area, while in 1980 the figure has risen to 37 percent.

Robert Collins, assistant business manager of the Plumbers Union Local 5, said that the union "has increased minority participation and employment opportunities, even in a period where [the construction industry] has seen a steep loss of jobs." Another official of the Plumbers Union cited figures that show that of the apprentices taken on by the union in the last four years, about 36 percent have been minorities.

The union representatives blamed the relative lack of minorities in the local building trades on nonunion contractors, who they say often import white tradesmen from the suburbs or from other states, and who rarely hire District residents except in low-paid jobs. The union leaders criticized the District government's policy of awarding city construction contracts on the basis of the lowest bid, with no consideration given to whether the firm hires union or nonunion labor.

But Courtland Cox, director of the city's Minority Business Opportunity Commission and an official of yesterday's hearing, said minority contractors often avoid the unions because hiring halls send them the least qualified workers. Cox said minority contractors charge that paying union wages would take them out of competition with large white-owned firms.

"The apprenticeship program is a farce," said black contractor William Shelton, who does not use union labor. "White contractors come to the District of Columbia and bring white labor. The black contractor is the one who takes the man off the street, out of the community, and teaches him a trade."

Harrell, president of HY-LO Industries, a metalworking and insulation firm, complained about the city's contracting process, which he said penalizes the small, minority contractor by bogging him down in red tape. "My firm has been able to find work in New York, in Maryland, in Pennsylvania, in Missouri, and yet I am unable to obtain a job here in the District," he said.