
In order to emit blue light, a semiconductor must have a band gap energy
within the region of 2.6 to 2.8 electron Volts. According to my physical
electronics prof, you can't get an LED with that band gap. That's why you
don't find blue LEDs or, for that matter, some other colour of LEDs. That
is not to say that blue LEDs can't be found.  I've seen 'blue' LEDs sold,
but they were just your typical visible light LED in a blue plastic covering.
They didn't emit very much light...

Source: Solid State Electronic Devices (Ben G. Streetman, 3d ed., Prentice
Hall)
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