In this Letter, ultrashort blue pulses spanning 350-500 nm are generated by combining the broadband frequency doubling technology with the two-stage multiplate continuum (MPC) generation scheme. We prepare relatively broadband input pulses and use a two-stage configuration for MPC generation, allowing us to employ thinner and less solid plates for further spectral broadening. Therefore, the deteriorations of the spectral phase, energy conversion efficiency, and beam quality, which occur more easily for 400 nm pulses, are effectively suppressed. After fine dispersion management, we obtain clean 5.2 fs blue pulses with a root-mean-square energy stability of 0.69% over one hour and excellent beam quality. Furthermore, lower than 8% energy loss during the spectral broadening process at each stage is achieved. The overall optimized performances and energy scalability of this blue pulse, as well as the possibility of further compressing the pulse duration, are likely to motivate more strong-field research with sub-cycle time resolution in this extended wavelength range.