Why use blue lights on your porch?
Blue light deter druggies from injecting themselves because they cannot see their veins so easily. Junkies keep away!
Blue lighting is sometimes used in public toilets (restrooms) to make it more difficult for drug users to inject themselves (veins are harder to see).
It was more difficult to see my veins through my skin, but there was normal-coloured lighting in the street outside, and one would assume that the users would thus just go outside instead, though the risk of detection is greater. (An additional result of the blue lighting is that, on going outside after spending more than a few seconds in the toilets, the daytime world appears much brighter and more optimistic, even on an overcast day: could retail designers or others make use of this effect? Do they already?) - A vein attempt?
As Timmy Jim points out in his comment people use blue lights to show support to fallen police in the line of duty.
Some people use them for the Fourth of July holiday:
Burke said his customers began asking for blue lights Wednesday. Since then, he estimates, his Home Depot store has sold between 50 and 60 blue lights. Normally, the store sells one to five a week. Usually, he said, residents want blue lights for parties or the Fourth of July holiday. - Area residents flipping switches to blue porch lights to back police
There is also the Blue Light Safety Project:
This is the Blue Light Safety Project.
A blue light signifies that this house is a temporary safe space,
open to anyone that feels in danger or threatened.
Knock on the door and the residents will provide support
until you feel safer or can make other arrangements.
If no one is home, you are welcome to sit on the porch
for a short time until you feel safe to continue on your way.
The project is an attempt to provide safety in our community outside of institutional solutions such as police. - Blue Light Safety Project
Or you could be like me. I use a blue light on my porch simply because I do not want a lot of white light coming in through the window!