I've observed phosphor LEDs age quickly, but not blue.  Absence of
data is not proof though.

I've found some biological coatings that like blue light, so check for
changes due to viewing angle, and see if the surface of the display
under the external filter has been colonised.

I also wonder if the UV from the LED has damaged the light guide
material in the device, or the external filter.

I would also check operating current per segment.  ;-)

--=20
James Cameron
http://quozl.linux.org.au/
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