The peeps above are right (apparently), the tooltip for nocturnal strike should show the current accuracy. It's 50% normal, 100% if the opponent is blinded, and 90% if you're also blinded.
If your pet is lower level than your opponent, then there's a 2% miss chance per level difference.
*edit* should also agree with elementals and weather. And note that darkness is a weather effect, so elementals aren't considered blinded for nocturnal strike, or other things that weather might do to pets.
If it's level 25 vs level 25, and there's no accuracy debuff, and you have an instance where a 100% hit chance ability misses, take a screen shot (showing the pet battle log) and post it to the Blizzard forums. There's all kinds of peeps there that absolutely do not believe that it can happen.