A picture would be helpful. Rabbitfish when stressed tend to get brown blotches and totally discolored. This would be normal. But without a pic there's no way to tell for sure.
if the size are blotches, then like Masterswimmer said, it's normal but if they are dots or some what one two millimeter, you should check if there are external parasites. I found that foxface healing from wounds could generate brown/black dots.