It wasn't actually a blood blister, just a sore made by the old boots I'm wearing. I didn't consider this as a possible cause before because I'd felt down with my fingers and pushed pretty hard all around, but couldn't feel any protruding nails or tacks, and I shook it out for any possible sharp pebbles, but nothing came out. Still, when I wore these boots, something, was wearing away at the area my sore was on. I put some cardboard soles in and it feels fine now. It's pretty much healed. I'd just never had anything like this before. Callouses yes, but sensitive sores, no.