Electrochemical machining (ECM) is an efficient technique that can produce complex structures. However, for a blade with platform, the common ECM method only considers the machining accuracy of the profile. Owing to the stray corrosion, a taper can appear on the surface of the platform and reduce processing precision. This study presents a novel method for improving blade platform accuracy in ECM utilizing a cathode tool with an auxiliary electrode (AE). Specifically, an AE is installed on a cathode tool to change the electric field and restrain stray current, contributing to greater machining accuracy. In this study, the simulations for electric field demonstrate that both the range and size of stray current are reduced and the predicted taper of the platform is reduced by the AE. Blade machining experiments were conducted for three different materials, namely, SS304, Inconel 718, and Ti-48Al-2Cr-2Nb (TiAl 4822). It was confirmed that stray corrosion of the blade platform can be better controlled by utilizing an AE. The optimal tapers of the SS304, Inconel 718, and TiAl 4822 blade platforms were −0.097°, −0.086°, and −0.080°, respectively, which represented reductions of 50.7%, 61.3%, and 77.6%, compared to the processing done by utilizing a cathode with an insulating layer.