William E. Brock, in his first speech as secretary of Labor, said yesterday that labor-management conflict "should clearly be a thing of the past" and that companies should adopt "progressive labor policies" if the nation is to be internationally competitive.

Brock, in a speech to the National Press Club, also made a strong pitch for the adoption of the proposed subminimum wage for teen-agers seeking summer jobs, describing it as a worthy "experiment" that could be a partial solution to the "tragedy of 43 percent unemployment among black teen-agers."

Brock, sworn in Monday to replace Raymond J. Donovan, sounded a conciliatory note toward labor unions, which had turbulent relations with Donovan and the Reagan administration.

Brock also took a concrete step toward improving relations with unions by appointing Stephen Schlossberg, 63, a former top official of the United Auto Workers, as deputy undersecretary for labor-management relations and cooperative programs.

The appointment of Schlossberg, UAW's general counsel and director of governmental affairs from 1963 to 1981, surprised several Labor Department officials, who said the choice was likely to irritate Republican conservatives who favor a harder line toward organized labor.

Brock praised unions at several points in his speech, saying, "Our unions are playing an increasingly constructive role in addressing the problems of new technology and job displacement. And they've championed our many freedoms, not only here but around the globe as well."

"The split between labor and management should clearly be a thing of the past," he said, "Companies with progressive labor policies are showing their competitive strength through increased productivity, and better labor-management relations. That means a better product at a better price. That means a sound future."

Brock cited the General Motors-UAW labor contract as an example of creative, cooperative solutions to problems. The pact includes a 5-cent-an-hour contribution to a retraining fund for dislocated workers, which Brock said would amass $50 million "invested in more productive Americans."

Brock also said unions must temper their opposition to measures such as the proposed reduction in minimum wage for youngsters aged 16 to 19. The measure would reduce the minimum wage from $3.35 to $2.50 between May and September for a trial period.

Reducing the minimum wage will make employers more likely to hire extra help, Brock said.

Unions have contended that the subminimum wage will induce employers to fire adults to hire cheaper labor, and that it will not create many new jobs. But Brock said it was time to "get off the dime" and experiment with the program.