Blue light is naturally occurring and is predominate in LCDs. It's at it's highest at noon outside. Even though you may not see a "blue" tint in your screen the blue light is still there. f.lux just reduces it.
f.lux does not work on non-jailbroken devices so if your device is not jailbroken then it may just be the calibration of your display.
I wasn't clear, the phone on the left exhibits a predominant blue tint that my 5s doesn't have. I am aware of color theory but the photo depicts an iPhone with a blue tint colorcast. My iPhone is balanced at white on 8.3.