In addition to F.lux i also use PangoBright now
http://www.pangobright.com/Been using it for a while.
During the day it's not always needed for me, but in the evening and night i definitely use it,
because then the light on my screen is too bright and hurts my eyes, especially
when there is a lot of white on the screen. I tried other programs and browser plugins,
but this seems to work best for me.
I think it's not only about blue light. Too bright light is probably bad for sleep as well.
(Don't forget to exit such programs if you do things like photo editing or take screenshots.)
And if you are looking for free software and alternatives, look here:
https://alternativeto.net/