The idea has been around a long time, since before Ferguson, and various accounts say it originated with a former officer, or the mother of a killed officer, etc. Probably there's no one origin but it's definitely not linked solely to Ferguson, though I'm sure someone did launch one campaign after that.
It's fine by me as long as no one starts to feel pressured by neighbors to have a blue porch light or be branded anti-police. It's a symbol of support, especially when there's been a police death or other incident, but I hope that the symbol doesn't distract people from more substantive support for police.
Our local police some years back said they really wish every house would have a porch light (blue wasn't mentioned) on all night, as well as motion sensor lights over back doors, garage doors, etc. They also asked for residents to be more proactive about phoning them on the non-emergency line if anything seems amiss and never to say, "Oh, I don't want to bother the cops"--it could turn out someone was casing the area or a person was in distress. Alongside blue porch lights as moral support, it would be great for citizens to ask police about ways like those to support more effective patrols in their specific neighborhoods.