Ich comes and goes on its own from my experience and more so with Powder Blue and Blue Hippo Tangs. I think 10 minutes is a little long for a freshwater dip. Usually 2-3 minutes is more than enough time. Also, when doing a freshwater dip, you want to make sure your fish is at least acting healthy, meaning swimming good and eating. This is very important, make sure the freshwater is RO or RO/DI water and you have the temperature and pH of the freshwater adjusted to match that of your tank. You will see the white spots start to fall off around 45-60 seconds.
Here is the problem with most "Ich" cures, it only kills one stage of the lifecycle. When you do a freshwater dip you are only killing the parasites that have come to the surface of the fish, but there are many more inside waiting to come out. So it will not cure the fish, but it will help reduce the amount of parasites on it, hopefully reducing stress and scratching on rocks that Ich causes. Freshwater dips are not done much because it is such a stressful thing to do to the fish and most people feel the risks outweigh the benefits. Something you will have to decide yourself.
White Spot/Ich/Crypto are all the same thing. Also, there will always be less visible spots on your fish in the morning than in the evening. You may only count 10 spots in the morning but 30+ in the evening. This is normal with Ich. The term "Ich" is a follow over from FW fish that get something similar that really is called Ich for short. On marine fish it looks similar and people just commonly use the term on the marine side as well. Others will correct you and say the proper term on the marine side is "white spot" for a common name. I guess Ich is the only incorrect term, but pretty much everyone knows what your talking about when you say it.
In the end, there are many things you can do to help with Ich, but there is no simple cure. You can do hyposalinity (most common course of action), copper treatment in a hospital tank (not recommended for a Powder Blue IMO), freshwater dip (again, not really something I would do on a PB tang, a yellow tang sure, not a PB though), add extra vitamins like Vita Chem and Garlic to your food (this is my remedy for small/early outbreaks), feed them good nutritious food like Rod's Food.