AJLang
Did you feel any discomfort on the site of your blister during the walk? If it came as a surprise when you removed your footwear, that suggests that the nerves in your feet are damaged (called neuropathy, for completeness). If you did feel it, then did you stop and try to adjust socks / apply plaster etc? I always keep a suitable plaster strip in my wallet, although always use it on someone else, then replace it for the next time. When hiking seriously, I usually apply zinc oxide tape to places I know may rub (back of heel with certain shoes). Main advantage of applying tape before blister forms is that it prevents skin getting broken and risking infection getting in. Changing shoes (or going barefoot inside if no neuropathy) is wise - you'll see people in all sorts of sandals after fell runs, adventure races etc, which I do, but it applies to any sore feet.