I recently experimented with 1000 mile socks - they have two layers, so one layer touches the skin, allowing the second layer to rub against the shoe or boot.
I was a serial blister former, and earlier this year, after doing 10 days on the Coast Path, covering about 22 miles a day, not a blister in sight!
Its friction between the deep layers of the skin, and the superficial top layers, at the junction between the dermis and the epidermis. This is made worse when the feet and socks get wet, and if the feet get hot.
I found the 1000 mile socks dealt with the friction, leaving my skin to get on with the walking ;-)
Before I used the 1000 mile socks, I used to have to protect my heels using carefully crafted adhesive semi-compressed felt (7mm) chamfered at the edges, with a cut out to accommodate my heels. My condition was made worse by having developed heel bumps (Haglunds Deformity) on both heels, caused )I think) by excessive pronation, leading to heel rocking. This encouraged the heel bone to 'protect itself', resulting in thickening of the bone.
I have no connection with 1000 mile socks, and I understand other twin walled sock are available. :-)
May be worth trying?
1000 Mile Socks