The black durgeon really does look like it's got velvet going on ...
Can't recommend treating in the display, as copper will be absorbed by rock, and to a lesser extent, by sand. That makes bringing the medication to a therapeutic level - and holding it stable there - difficult. Afterward, you're faced with the challenge of removing the meds from the rock, lest they leach out at some later date and cause problems. You've received some excellent advice on treatment above!
I can't really see the black spots on the tang in the photo - if they're small and sharp, almost as though someone spilled pepper, then "black ich" (turbellarian worms) may be a possibility. Prazi-Pro usually handles them.
If there is velvet in the tank, it won't be sufficient to remove and treat the trigger - all the fish will need to be treated.
~Bruce