Representatives of a black government employes association told members of Congress yesterday that they oppose any revision of the existing veterans preference law - a law that the Carter adminstration has called a hindrance to the hiring and promotion of minorities and women in the federal civil service system.

In the second day of hearings on Carter's legislative proposals to over-haul the civil service, officials of the Nationsl Alliance of Postal and Federal Employes described efforts to curtail veterans preferences as part of a "backlash" against what some consider a "disproportionate number of black veterans."

Rep. William Clay (D.Mo.), who is black, challenged the Alliance position, saying, "Testimony to this point, other than from veterans' groups, has indicated that veterans preference has been abused and that it has not benefited blacks and has worked against women" in federal hiring and promotions.

The Alliance officials also said they are against the removal of any existing job safeguards for federal employes, and they termed some of the president's proposals a "power grab."

Their testimony questioning by members of the House Post Office and Civil Service Committee explored such issues as: how to measure the "merit" or performance of workers, especially in higher-level jobs, without the private sector's "botton line" profit motive; How the percentage of employes fired by the federal government compares with that for the private sector; and - a fundamental conflict of civil service reform - how to increase management flexibility and efficiency while still protecting workers against political abuses and even against the fear of such abuses.

During the discussion of veterans preference, Alliance official John White noted the high number of black serving in the all-volunteerment figures among blacks nationally. He suggested that veterans prefer-volunteers when they returned to the job market.

Clay, however, pointed out that those who joined the volunteer Army after October 1976 are in any case not eligible for veterans preference.

The president's proposals would cut back to 10 years the lifetime advantages for nondisabled veterans while strengthening the benefits for disabled and recently returned (Vietnamera) veterans, according to officials.

Roy Ash, chairman of Addressograph-Multigraph Corp., and former director of the Office of Management and Budget in the Nixo n administration, told the commistrators need more "flexibility and discretion" in order to get their agencies to perform well.

"Over the years, a paralyzing imbalance has evolved that favors the welfare and security of employes over getting the job done well, at least cost, with best results," he said.

Ash and another executive reaf-firmed an earlier endorsement of Carter's plan by the Committee for Economic Development (CED), of which they are trustees. CED is a nonpartisan group of business leaders, many with experience in high-level government posts.

The series of hearings on civil service legislation is scheduled to resumeearly in April.