digital - Why is the blue channel the noisiest?

It is widely observed that the blue channel in digital cameras is the noisiest. I've certainly noticed that with my camera. Why is this?

Is it an artifact of some particular technology (e.g. Bayer array or CMOS sensors), or is something to do with the physics of higher-frequency light, or is it related to human vision?

_{Followup question: Why are sensors less sensitive to blue light?}

Answer

In addition to the sensor response discussed by Tall Jeff, most scene illumination (sunlight, incandescent) is deficient in blue light relative to green and red. Fire up this Java blackbody simulator and see that blue is lower than green or red for color temperatures of interest (~5500 K daylight, ~3000 K incandescent).

There's another small factor that compounds the problem. CCD and CMOS arrays are photon-counting detectors. Most plots, including those in the blackbody simulator above, show spectral energy density, not photon counts. Blue photons are more energetic than red photons, by the inverse ratio of their wavelengths, so for the same energy value on the plots, you would get about 25% more red photons than blue photons. And that's the starting point for the sensitivity effects Tall Jeff describes.

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Regarding CCDs and backside-illuminated sensors, frontside-illuminated CCDs do suffer from the same diminished blue sensitivity, as much of the blue light is absorbed while passing through the non-sensitive gate structure of the chip. Backside-illuminated sensors will see an improved blue response. See this typical spectral response curve (for various types of research-grade CCDs).