Onboard LEDs

#include "Arduino.h"

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(LED_BLUE, OUTPUT);
}

void loop() {
  digitalWrite(LED_BUILTIN, HIGH); // ON
  digitalWrite(LED_BLUE, HIGH); // OFF
  delay(500);
  digitalWrite(LED_BUILTIN, LOW); // OFF
  digitalWrite(LED_BLUE, LOW); // ON
  delay(500);
}