We have carried out a set of experiments using anhydrous carbonaceous particles to determine the effect on the electrorheological properties of blending particles of different conductivities. We find that there is a significant dip in the shear stress under an electric field as the concentration of higher conductivity particles is increased, showing that uniformity of the electrical properties among the particles is a most important factor in achieving optimum electrorheological fluid (ERF) performance. We explain this behavior by a simple model based on the strength of particle-particle interactions. Measurements of the dielectric permittivity show that the cr parameter of the Cole-Cole expression provides an excellent way to express the degree of uniformity in the electrical properties. This gives a convenient method to evaluate the potential performance of an ERF.