This is probably another sport, like cycling that you wouldn’t expect to get foot blisters but they happen too and usually at the back of the heel which is boot related. When riding a horse, a person’s weight is put on their heel area which means the back of the foot is moving but the boot which is usually very stiff, stays still and with any movement like this, friction/pressure/is created and the consequence is blisters.
In horse competitions the long boots that are worn are particularity stiff and most riders only wear them whilst riding and then remove them once on the ground. Prevention is key here, usually the longer riding boots have long zips which means access into the boots is easy to apply Engo patches on the back of the boot where contact with the back of the heel is causing the blisters
Wicking socks will help to create a moisture free area of the foot, which will keep the feet warm in the winter and cool in the summer.
Armaskin liner socks would also work well under your normal socks as these reduce friction on all aspects of the foot.
Achilles gel sleeves that are used for skiing and ice-skating will work really well in this situation too, also Epitact gel oval patches for the skin can be re-used a few times helping to prevent blisters especially for competition events these will need securing to the skin with hyperfix tape.
Taping with hypafix or KT pro tape as a blister prevention measure with 2toms blister shield powder.