That you are a werewolf
My friend did this once to me,
Your "friend" turned you by biting/scratching you
now I am eternally blind, you see.
Werewolves are said to be color-blind. Also, in the series Teen Wolf, some wolves have infrared vision while others are color-blind(Alpha and Beta wolves).
I can't look at the sky at night.
It just gives me such a fright.
It brings back the memory,
of what he did to me,
Not sure about this part, you could be traumatized and be afraid of the night sky/full moon.
And only when it is full,
Will I be able to have a midnight stroll.
You turn into a werewolf at full moon and do stuff werewolves usually do