The theological and electric properties of blends of poly(ethylene terephthalate) (PET) and high-density polyethylene (HDPE) filled with various types of carbon black (CB) were analyzed in detail in this project. Four types of CB samples with available values of surface area, particle size, porosity, density, and maximum packing fraction were considered. Blends were prepared using an internal mixing chamber at two different rotational speeds, prior to mold compression of the samples. The rheological properties of the blends with varying polymer composition and a constant amount of CB were recorded in terms of torque variation with time for two shear rates (in terms of rotational speed). Rheological data were related to the resistivity of blends. Results show that the CB structure (porosity, surface area, apparent bulk density, and particle size) largely determine the resulting equilibrium torque and electrical properties. Furthermore, since CB is preferentially located in the HDPE phase, higher conductivity is observed as the PET content decreases, since the relative CB content in this phase increases.