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I still have a stack of small round black stickers for the purpose of reducing the brightness of (mostly) blue leds. On some devices I had to stack 2-3 of them to reduce the amount of light enough.

A little kapton tape tints it to a less irritating and warmer tone.

Same here, and the green leds on my Thinkpad get the same treatment.

I glued stabilo pen caps over those leds - instant goa uld spaceship fuse box look

IIRC that was because blue LEDs used to be more expensive than other colours, so it was (very briefly) an indicator of high price.

The inventors of the blue LED won the Nobel Prize for Physics, because it was so difficult to make, and paved the way for white LED lighting and lights with tuneable colours:

http://www.extremetech.com/extreme/191580-blue-led-inventors...

They are still more expensive - a search on Mouser yields many 6-cent red LEDs and 10-cent green LEDs but no blue LEDs under 15 cents. But yeah, in the past the difference was muuuuch bigger.

You still see them in car tuning, specially underneath.