If they had been stored in a hot loft or damp shed it might have explained it but there shouldn't be any problem if they've been stored in the house at normal room temperature.
It depends where your blisters are but they are usually a sign that the boots are poorly fitting and usually shows up as either blisters on the heels caused by heel lift or under the ball of the foot or toes caused by your foot sliding backwards and forwards inside the boot or if the blisters are on the outside of the little toe it would be an indication that the boots are not wide enough for your feet. It could even be something as simple as wrinkles in your socks going unnoticed.
The problem can usually be cured by either tying your laces in a different way to lock your heel into the back of the boot (plenty of videos on YouTube showing this), wearing a thicker sock to take up any excess volume inside the boot or by adding a volume reducer under the insole.
Having said all that if you managed the PW in them without a problem I'm out of ideas but something must be different or have changed to be suddenly causing the blisters.
I came across this article which may be worth reading and considering if the insoles in your Scarpa boots are the usual cheap manufacturer supplied low quality ones
https://andy-kirkpatrick.com/articles/view/sole_powerI didn't buy the Lidl bag as I've already got a cupboard full of bags of various sizes.