I buy boots that fit. They usually are my normal size, but occasionally 1/2 size larger for some manufacturers. I buy size 11 insoles for size 11 1/2 boots.
I think that a lot of blisters on soles come from buying boots that are too large. One's foot slides back and forth. Blisters along the sides may be from boots that are too narrow. Black toenails often are from boots that are too short; on downhill stretches, the nails hit the end of the boot repeatedly. If the boot is the right fit, then loose laces may allow the toes to impact the end of the boot.
Blisters under toes may have a lot of causes. I concentrate lubrication on the top and bottom of my toes, and it prevents the heat of friction on toes. Blisters between toes are a curiosity to me. They may come from angling a foot sideways in response to fatigue or pain; that is, changed body mechanics. They may also come from boots that are too wide, so that the toes develop a mind of their own!
I counsel against automatically buying boots that are larger than normal. Boots should fit, and that requires trying them on, not adding a digit to the normal size.