Can you post a picture of him?
Has this been going on for awhile or did it just happen?
A couple things can cause them to go dark. Stress, pain, sickness, hormones, fear, or aggression. Do any of those help explain what might have happened?
If it's just a one time thing and it's over with quickly, it's usually just because they got scared, or were trying to act tough for one reason or another. If it's constant it can be an indicator of some of the other things I mentioned.
Did he get scared or go through a scary experience?
If you want, we can go over your husbandry too to make sure there is nothing wrong in that aspect that could be affecting him.
-Brandon