I was told, though I haven't confirmed it yet, that the blue tangs don't have scales so they are more vulnerable to ich.
I doubt it's much of a "looking the wrong way" thing as much as it is just a general stress problem with them.
I guess as a safe bet you could put one into a quarantine tank and treat it with a copper solution (NO
CORALS OR INVERTS) and that should take care of any ich.
That doesn't exclude your main tank not having "dormant" ich though. Perhaps that's not even possible. Can someone clarify if ich can be dormant?