You can test the negative easily, but the positive is harder. Thus: placebos.

On the level of the individual a working placebo is a success.

You're saying that my eyes straining going away from reduced blue light is placebo? I can feel it right away and it gets worse in minutes, time and time again. As soon as I remove blue light the strain is gone. Honestly, I don't care what other people have to say, to me it's obvious that it helps and I stick to it. Again, I don't think this is universal and it may not help you if you don't notice immediate improvement.