A monolithic KNbO3 resonant cavity was used to generate 455 nm radiation by sum frequency mixing of a 1064 nm Nd:YAG laser with resonantly enhanced 795 nm emission of a diffraction-limited externally injection-locked GaAlAs broad-stripe laser diode. Blue output of 154 mW was measured with 4.5 W of 1064 nm and 240 mW of 795 nm radiation in quasi-cw operation.