The blue light is an indication of FOD (foreign object detection) and this is a security guarantee that must be included in order for Flash to receive Qi certification.
When a product has a FOD, it means the transmitter of the wireless charging system can detect if a foreign object, such as metal, is on its surface. In this event, the wireless charger either stops charging or does not start charging at all.
When Flash detects a foreign objects that is considered abnormal, the indicator light will flash at a high frequency to alert you to remove the foreign object.
If Flash did not have a FOD, it would continue to heat up causing the object placed near the wireless charger or between the charger and the phone (such as the phone case) to become very hot. This could lead to potential damage to your device or injuries.
The FOD function is not only necessary for Qi certification but also provides a better guarantee for your safety.