On the last two planes I took, just last week, there was the control shown in this (very blurry, sorry) photo. The planes were Boeing 787-8. It seems to be made of two half-circle buttons.
On the first plane, pressing the half buttons moved the blue light up or down. On the second, nothing happened. Nothing else seemed to change. I paid attention to the window and cabin light but nothing happened. Since these planes had no apparent window shade, I thought the control would operate a mechanism to act as shade.
What is this control for? What is doing something? Could it have been not operational on one or both planes? Seems an unlikely coincidence, particularly since the behavior of the blue light was different.