We present extensive numerical studies to determine the phase diagrams of cubic and hexagonal blue phases in an electric field. We confirm the earlier prediction that hexagonal phases, both two and three dimensional, are stabilized by a field, but we significantly refine the phase boundaries, which were previously estimated by means of a semianalytical approximation. In particular, our simulations show that the blue phase I-blue phase II transition at fixed chirality is largely unaffected by electric field, as observed experimentally.