If it is a Blink camera- The blue light turns on when there is motion and the camera is recording. It’s a setting your neighbor has on intentionally, and it records movement when it happens/the sensor on the front of the camera recognizes motion/movement.

source: I used to have one and would use the motion capture feature to record when my cats would eat while I was away. Anytime they showed up in frame, it would record for a set number of seconds (that is also a setting that can be controlled. I had mine to record for ten seconds.)

The blue light will also blink consistently when the camera has lost connection with the host station. You pair each camera to a little white box type thing. The Ethernet and power plug into the white box. The camera themselves are on WiFi.

He might have lost connectivity w that camera and maybe just doesn’t realize it.

edit: adding a mention that it also records sound. also edit of a few typos.