If it's in that bad of condition, it will take some time for it to get better. Do you have a quarantine tank to keep it in? Hate to find out it has some disease and hurts the rest of you live stock by keeping it in your main tank. Main thing to watch out for with blue tangs is they are very prone to ICH when they are stressed. Make sure it has plenty of hiding places as they like to hide, sometimes laying on their sides under rocks. Fortunately they tend to eat just about anything. Out of all the tangs, they tend to eat more meaty foods rather than algaes/seaweeds.