It might be best to just let it grow back on its own. If its from the blanket, a sleezie to protect the chest from being rubbed raw is an idea. You could also use MTG if you want to use something. Its a great hair growth product.
Well if she is still wearing a blanket when you buy her then you should get one of those sleezy sholder guard things to protect her sholder. Or maybe if the blanket doesnt fit and thats the problem a new blanket would fix this. I have used MTG and it does work for regrowing hair but i thought that it also dried out my horses skin. Her skin does get dry in the winter so it could have just been adding to that but for the hairgrowth part it did work.
MTG works absolute wonders. You see a visible difference in just a couple of days! It is infamous for sun bleaching and sunburn but I haven't had any of those problems with my two horses (a bay and a red dun) because I use Santa Fe detangler along with it which is a great detangler and conditioner that also has sunscreen in it. My bay's black tail is as black as possible and I use MTG weekly. Neither horse has been sunburnt or anything like that.
It is also very greasy. The smell kind of sticks on your hands. I use soft scrub with lemon to get the smell off and it seems to help. Because of how greasy it is, it's also good to wash out the hair a little bit more often than you normally would to keep it from getting too icky.
It really does work wonders, though. My horse's tail was pretty damaged from rubbing because he was wormy and neglected at his previous home, and all of those short pieces are really filling out and it's looking just beautiful. I would definitely give it a try. You can use it once a week as prescribed on the bottle, or I've also heard people say they use it every other week or once a month and still see noticable results.