Unfortunately crypto/blue tangs go hand in hand. I don't know anyone that has a blue tang that hasn't had crypto at some point.
It isn't sounding good for the fish. A suggestion if possible would be to soak some seaweed/nori sheets in something like Garlic Guard/Garlic Xtreme and/or Boyd's Vita-Chem and hope it will eat some of that to get its strength back.
Think of it in human terms.. when we get sick or stressed out, one of the first things that happens to us, just like the fish, is we lose our appetites. We are smart enough to realize that even if we don't feel like eating we still need to, unlike fish who will starve themselves to death if the parasites don't kill them first.