PSU4ME:
They are not the same fish, but are mistakenly named the same common names. I have had a Acanthurus japonicus, which is what I have tipiclly refered to as the powder brown. I ahve never actuallys een a nigricans in my lFS stores, and I believe they are somewhat harder to find too.
Mine eats very well, but came down with ich shortly after getting it. It actually gets bullied a bit from time to time by other fish in the tank like the achilles tang. This is more the rarity and not the rule. Despite both of them having similar body shapes, they have both done well over time.
They are very active swimmers, even mor so than all my other tangs (yellow, naso, achilles, and hippo). They are also very sensitive to ich, and even more so than any of the other tangs mentioned. I think the powder blue is the only one ahead of him in that department. If I was to have this fish in a 90 gal, which with all the rock, sand, and corals is going to to be tiny for him in 6-9 months, I would keep him by himself. Will do better in the long run.
Knowing what I know now, this and a moorish idol are the only fish I would quarentine.