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.