Blue is not a commonly occurring flower color and often tends to look purple instead of a true blue. Using flower color, blue in particular, as an outward symbol of your support for a cause would have many complications--variety, climate zone, and duration of bloom. This doesn't even call into play the possibility of color mutation. GASP! What if pink or white, common mutations, signified your dislike for the Purge and made you a target instead?!
Synthetic flowers on the other hand would be far more useful in this situation and could be used as weapons, hide security cameras, or dispense poisonous gas sprays. Honestly the whole security system seemed a bit antiquated--no safe room, no flame-throwing door decorations, no remote controlled gun turrets protruding from the roof?! Blue flowers. Just blue flowers to signify a symbolic thumbs up to ward off the jerks who take advantage of this one night.
The kid had the best security within the home--remote controlled mobile surveillance and a cleverly hidden safe room. The security professional, the dad, couldn't even keep his kid from using the code to open the fortifications. I would also like to add two more things about the security plan...1) a double secured entry would have made more sense, and 2) the ability to open only ONE entrance at a time rather than the whole doggone house at once.