A blue light bulb is often used on homes where the residents do not smoke. its a "smoke free and proud" kinda thing. dont know where I know this from but I know Ive heard it before.
the light on my porch is yellow
A retired Marine down the street from me has a purple yard light in his yard. He does it just to make his yard stand out from the rest of the neighbors with white porch or yard lights.
I have a yellow light and it means "bugs stay out!".
However, in all seriousness, if you 'google' Project Blue Light; you'll find the blue light is displayed to show honor to fallen LEOs and to promote peace. Not a bad idea.