We have extensively studied the frequency noise and relative intensity noise spectra in a tunable external-cavity InGaN diode laser at blue (420 nm) wavelengths. We report flicker (1/f) frequency-noise behavior at low Fourier frequencies measured using offset frequency-absorption spectroscopy on Rb85 vapor cells, which yields an estimated lasing linewidth of 870 kHz. From considerations of high-dislocation density in III nitride epitaxy, 1/f noise and linewidth were expected to be larger than in conventional III-V lasers. Surprisingly, the measured noise characteristics are comparable to or better than those of near-infrared distributed feedback lasers and external-cavity diode lasers. The noise-reduction mechanism is attributed to the wavelength dependence of 1/f noise. We discuss challenges in atomic spectroscopy applications caused by defects and mode-clustering effect in GaN lasers. Using the Hakki–Paoli analysis in an aged laser diode, we provide possible explanation about the origin of observed mode clustering.