Blink only one led in a led strip
Everytime, that you want the LEDs to change, you need to call the FastLED.show() function. You are doing that only once after you set the values multiple times. Also you need 2 delays in total (1 for the LED being on, 1 for it being off).
The workflow for blinking an LED would be:
Set LED to on
Call show function
Delay
Set LED to off
Call show function
Delay

You need to translate that to code.