I am currently using Seeed nRF52 mbed-enabled Board v1.0.0 and bluefruit.h library.
When trying out the adv_AdafruitColor.ino sketch, the onboard blue led blinks.
(Examples for Seeed XIAO nRF52840 Sense > Adafruit Bluefruit nRF52 libraries > Peripheral > adv_AdafruitColor)
Anyone knows the reason for this? I might have missed something out but I don’t see any code indicating writing to the led / turning the led on and off.
Well that usually means it advertising the BLE , but I haven’t opened that example yet.
The Blue LED is the default color too, I believe?(LED_BUILTIN) generic.
If its flashing and connected when it’s solid on most of the examples I’ve seen.
HTH
GL 
Found it ,From the header,
also look at the Serial port monitor it should give you some output.
/* This sketch demonstrates the Bluefruit.Advertising API(). When powered up,
ble.start_scan(AdafruitColor) in CircuitPython will hear@Rhys3logy Blue LED flashes to let you know that bluetooth is advertising. This feature can be turned off by function call. See Bluefruit.h.
// Don’t flash led while advertising
Bluefruit.autoConnLed(false);
You can turn it back on by passing boolean true.