Why are blue badges blue?

Last night, I watched a sliver of a BBC programme Parking Mad, documenting the endless angst experienced by the denizens of the capital in their quest to find parking spaces that don’t come with a health warning. In the main, the doc was good, attempting to show the story from all points of view; those of the drivers, wardens and adjudicators when fines are in question. However, I was left with the feeling that design issues could be making life difficult for all involved. There was one nice old gentleman who found himself with a £70 ticket when a traffic warden failed to spot a “blue badge” on his dashboard. Photographs taken of the car by the warden were indistinct, failing to reveal that the driver’s blue badge was not on display. For this reason, the adjudicator waived his fine and he went away happy. A numbskull, non-driver like me has a question: why does the blue badge have to be blue? After all, blue is a difficult colour to see through glass, even on the finest of days – what happens when it is raining or foggy? Why cannot the blue badge be red, or pink, or yellow, or even fluorescent orange? Authorities, over to you.