Most of the time, black triggerfish look like the lower photo, a fairly uniform black, apart from two bright pale blue lines at the base of the dorsal and anal fins. However, when they become aroused or agitated, their colors can change, and this color transformation can happen very quickly. Sometimes it’s just the brilliant blue lines radiating from the eyes, sometimes the flush of yellow or orange on the sides, sometimes the blue-green lines along the side.
This black triggerfish gave me the full display, and rather than swimming off, it hung around and presented a broadside view. It’s possible it was defending its territory or perhaps some eggs.
I took the photo and swam on, not wanting to bother the fish more than I apparently already had.
Graham's Island 
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Interesting to see it in both states. How nice of you to move on so as not to bother it further.
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I think they get used to people to some extent, as there are lots of divers and snorkelers here. Also, fish in areas where fishing is prohibited seem noticeably less bothered by human presence.
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It is interesting that sea creatures can become accustomed to non-hunting humans visiting underwater and so relatively tame. Of course those species that are very defensive of their nests/young will always be on guard!
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