What is blue noise?
Blue (or azure) noise is a random signal whose power rises 3 dB with each octave - the exact inverse of pink noise. Where pink tilts warm, blue tilts bright: most of its energy lives in the top octaves, giving a fine, sizzly character often compared to a gentle spray or hiss of steam.
Its famous property is in the mathematics: blue-noise distributions place energy (or points) evenly with minimal clumping, which is why the shape appears in audio dithering (noise-shaped quantization pushes error into blue-ish spectra where ears are less bothered), in printing halftone patterns, and in computer graphics sampling.
What blue noise is used for
Practically: testing tweeters (all the energy sits where only the tweeter plays), masking high-pitched annoyances like electronic whines that darker colors cannot reach, and A/B-ing against pink noise to hear what "spectral tilt" really means - play both back to back and the concept clicks instantly.
As a comfort sound it is a minority taste - most people find sustained treble energy fatiguing at volume - but at low level some prefer its lightness to white noise's density. If you want brightness with more body, white noise sits exactly between blue and pink.