In general Not later than 6 months after the date of the enactment of this Act, the Secretary of Labor (or the Secretary's delegate) shall modify the regulations under section 404 of the Employee Retirement Income Security Act of 1974 ( 29 U.S.C. 1104 ) to provide that, in the case of a designated investment alternative which contains a mix of asset classes, a plan administrator may, but is not required to, use a benchmark which is a blend of different broad-based securities market indices if— (1) the blend is reasonably representative of the asset class holdings of the designated investment alternative