I'm not sure i quite understand your question. Especially the "random on it's own" part.
If what you want is a LED that's blinking for random lengths of time at random intervals you could use the Delay() function both with random "on time" and random "off time" something like:
turn led on
delay("random ms")
turn led off
delay("random ms")
EDIT: if you also want it to "flicker" you could use a random PWM value