There is a dependency between the battery voltage and the remaining capacity (level) of a battery. This was programmed to give the user a hint to replace the battery.
Why blue LED is not ON during advertising green LED blinking at BLE_Heart_Rate_Sensor ?
Somehow ,I confirmed Blue LED is ON after connecting state.
The blue LED was designed in this project (if I have the same) to signal any notifications received. This is more general than only advertising etc.