We report a new source of cw, single-frequency radiation in the blue, offering extended tunability and practical powers in a compact, all-solid-state design. The device is based on a green-pumped, cw, singly resonant optical parametric oscillator using $\mathrm{MgO}$ -doped stoichiometric lithium tantalate ( $\mathrm{MgO}:\mathrm{sPPLT}$ ) as the nonlinear material. By internal second-harmonic generation of the resonant near-infrared signal radiation in a $5\mathrm{mm}$ ${\mathrm{BiB}}_3{\mathrm{O}}_6$ crystal, we generate nearly $450\mathrm{mW}$ of cw, single-frequency blue power over a tunable range of $425–489\mathrm{nm}$ with a linewidth of $8.5\mathrm{MHz}$ and a Gaussian spatial beam profile. The demonstrated wavelength coverage can be further extended by using alternative gratings for the $\mathrm{MgO}:\mathrm{sPPLT}$ crystal.