<p>The rheological behavior of immiscible polymer blends is significantly influenced by their interfacial structure. In this study, the low-frequency tail behavior observed in the Cole–Cole plot of bi-polymer blends was analyzed to establish a theoretical model. The model introduces key parameters, including the effective bulk modulus at the interface, interfacial length, and diffusion interruption coefficient, to describe the tail behavior. The validity of the theoretical prediction was ascertained through rheological experiments of polypropylene and polystyrene blends prepared under varying processing conditions, accompanied by complex nonlinear least squares analysis. The results of the study indicate a correlation between the slope and extent of the tail in the Cole–Cole plot and interfacial morphology, diffusion characteristics, and mixing parameters. A more pronounced tail was observed in co-continuous morphologies and the case of enhanced interfacial interactions and diffusion. This study proposes a novel rheological criterion for characterizing interfacial structures in polymer blends.</p>