Using self-consistent field theory (SCFT), we explore the phase behavior of a diblock copolymer (BCP) melt in an applied electric field, with different dielectric constants assigned to each monomer type. The electric field penalizes the interfaces between species domains that are not parallel to the field. Under the present mean-field approximation, lamellar and cylindrical structures reorient to align their interfaces with the electric field, such that these mesophases will have the same electrostatic free energy contribution as the mixed (disordered) state, and their relative stability will remain unchanged. In contrast, sphere and network phases do not have an axis of dielectric uniformity; consequently, the preferred orientation and morphological response of these phases must be determined numerically. We compute the phase diagram for a BCP melt in the presence of an applied electric field by comparing the free energy of each phase at its thermodynamically preferred orientation relative to the electric field vector. We find that the stability regions of the sphere and network phases shrink with increasing field strength, in favor of the disordered, cylindrical, and lamellar phases. Moreover, the double gyroid network phase is more strongly disfavored than the orthorhombic Fddd network phase, such that the predicted region of stability for the Fddd phase is shifted to larger segregation strength (lower temperature).