Optical activity measurements in the blue phase of cholesteryl decanoate are presented for three different visible wavelengths. The optical activity of the blue phase is reduced from its value in the cholesteric phase, approaching zero nearly continuously at the blue phase to isotropic liquid transition. In agreement with previous work on the blue phase, the wavelength dependence of the optical activity indicates that the blue color of the phase is due to a region of anomalous optical activity and selective reflection at approximately 450 nm. The results also provide evidence for two distinct blue phases in this cholesteric liquid crystal.