FASTLED fade to backby

void loop(){

 for(int i = 0; i < NUM_LEDS; i++) {
            leds[i] = CRGB::Blue;
            FastLED.show();
            delay(30);
  }
for(int a = 0; a < NUM_LEDS; a++) {
    leds[a].fadeToBlackBy(64);
    FastLED.show();
    delay(30);
  }
}