Concentration fluctuations and phase transitions in multicomponent polymer blends comprising polyethylene (PE), head-to-head polypropylene (PP), and a symmetric PE-PP diblock copolymer were studied by a combination of light and neutron scattering. When block copolymer concentration was below a certain threshold (12.5 vol % in our system), the blends exhibited macrophase separation. At higher block copolymer concentrations, we found stable, periodic structures that were characterized by a neutron scattering peak at finite scattering vector, q. In this regime, the block copolymer serves as a surfactant. At high block copolymer concentrations, ordered microphases that were similar to those found in pure block copolymer melts were obtained. At lower block copolymer concentrations, however, microemulsions that are periodic phases that lack long-range order were obtained. Concentration fluctuations of individual components in these multicomponent mixtures in the single-phase regime were examined by conducting neutron scattering experiments on contrast-matched systems. The formation of disordered phases and microemulsions is announced by transient intermolecular aggregation of the homopolymers. In contrast, the formation of ordered structures is announced by both intermolecular and intramolecular signatures. Many of our results are in quantitative agreement with theoretical predictions based on the random phase approximation.