My suggestion is buy some live brine shrimp and Entice Appetite Stimulant and soaked the brime shrimp for 5 minutes and the tang should be able to eat the food. This will help boost their appetite and your blue tang should eat whatever you feed after three to week. My LFS suggested that idea and thats what I did on my blue hippo. I soaked the brine shrimp everytime a feed him and after a week my hippo tang eat whatever I feed it. Also what I did was I bought garlic extreme or garlic guard and everytime I feed my yellow, blue and purple tang with brine shrimp I soaked them for about 5 minutes and my tangs goes crazy and start chasing the food. This helps them boost their immune system that IMO.
Good Luck and let me know how it goes.