Our expert says:
Flat feet sometimes cause blisters on the base of the foot, under the arch, but not usually at the back of the foot. That sounds more like something where the foot is catching on the heel of the shoe, probably because the shoe size is not ideal. Now, I don't know what process you followed getting the shoe, but all the problems she's had seem related to size. Blue toenails and blisters are also usually the result of shoes that are the wrong size, causing the foot to slide in the shoe and strike the front. So that seems likely option number 1.
The other option is the socks. Remember, blisters are caused by heat, which is caused by friction. So some socks trap heat, and allow blisters to form far more easily than others, and perhaps she just needs to look at her socks - that would be the easy answer!
Lastly, what she might try is to rub vaseline on the affected areas, but before she walks. That helps to reduce the friction as well. Oh, and one more thing, don't give up on the new shoes yet - new shoes almost always cause blisters and it takes a while before the shoe is "soft" enough to be used without causing blisters.
So perhaps give the shoes a little while to wear in.
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