Well, at this point since he's already in the tank with the other fish and if it is ich that you're seeing, the tank is already infected. If he only has a few spots, I would wait and see how things go. IME, healthy, well-fed marine fish can often fight-off a mild case of ich on their own. If things are looking worse as the week progresses and other fish are not showing any symptoms, your best bet would be to setup a hospital tank and treat him. My preference for treatment would be hyposalinity (if you do a search here it's been discussed a lot).
Tangs need a lot of food to stay healthy, so IMO your best bet at this stage is to make sure he's getting adequate nutrition. I'm not sure what species you're referring to when you say "blue" tang. If it's an Atlantic Blue Tang (A. coeruleus) or a Powder Blue Tang (A. leucosternon) I would plan on feeding nori on a clip daily in addition to regular feedings. If it's a Pacific Blue Tang aka Hippo Tang (P. hepatus) they tend to go more for meaty foods so I would make sure to feed some nutritious meaty stuff like frozen mysis (you can also soak the mysis in something like Selco to further enrich it). For the latter species, you could also try offering nori but the fomer two are more predominantly herbivores by nature.
BTW, if your tank is the popular glass 65 gallon that is 3' long, IMO it's not really big enough for any of those species long-term. If it's a 4' tank, I think you're OK with the Atlantic Blue, but the other two species mentioned should really be in a 6' tank for the long-term IMO.
HTH