A balanced A-C diblock copolymer surfactant was used to organize mixtures of immiscible A and B homopolymers. The C block of the copolymer exhibits repulsive and attractive interactions with the A and B homopolymers, respectively, leading to rich phase behavior. Experimental results indicate the existence of a microphase-separated state at low temperatures, a homogeneous phase at intermediate temperatures, and macrophase separation at high temperatures. It is unusual for a microphase-separated blend to exhibit a homogeneous phase prior to macrophase separation. In this study, component A was saturated polybutadiene with 89% 1,2-addition, component B was polyisobutylene, block A of the diblock copolymer was chemically equivalent to component A. and block C of the diblock copolymer was saturated polybutadiene with 63% 1,2-addition. We use a combination of Flory-Huggins theory (FHT), self-consistent field theory (SCFT) and the random-phase approximation (RPA) to understand the origin of our observations. All of the parameters needed for the SCFT, FHT, and RPA calculations were obtained from independent measurements. The measured length scale of the periodic concentration fluctuations in the homogeneous state and the domain spacing of the microphase-separated blends were in close agreement with RPA and SCFT, respectively. The transition temperatures between phases predicted with theory were in reasonable agreement with the experimental measurements.