Hornreich et al. predicted that blue phases I, II, or III should undergo structural transformations into anisotropic uniaxial blue phases in electric or magnetic fields. It is shown here that, besides a hexagonal blue phase BP $H^{3\mathrm{D}}$ discovered previously, a novel blue phase, called BP X, can be obtained either from BP I for E∥[110] or from BP II for E∥[100]. Using the method of Kossel diagrams, clear-cut evidence is presented that blue phase X has tetragonal symmetry. The transformation of BP I into BP X involves a homogeneous strain in the plane perpendicular to E and a dilation in the field direction.