Was the fish clean before and then these spots showed up all at once or did they come slowly one by one?
Are the spots slightly raised?
Does the fish behave like something is bothering him? Any scratching, heavy breathing or erratic swimming?
Black ich usually has no well defined waves like "normal" ich. Also black ich spots are usually relatively well defined.
I'm not an expert on naso, but this could be a stress coloration or the way this fish reacts to ich or other skin irritations.