I've had my blue for a couple years and have never seen anything like that happen. :-(
Following to see if someone else chimes in with the actual answer.
I would say to watch it though, turning colors isnt usually a good thing, anything out of the ordinary with a tang should be watched closely.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.