The dynamic dilution theory of stress relaxation, quantitative for star polymer melts, cannot be directly applied to star-linear blends. The linear chains on their reptation time scale tau(d) release constraints on the star arms, resulting in constraint-release Rouse motion for the star arms not described by dynamic dilution. We present a microscopic theory without adjustable parameters for stress relaxation in such blends, which is in excellent agreement with dynamic rheology data for the full range of blend fractions.