In the original post you stated the blisters were forming in between your toes. Socks may not help. I do a fair amount of backpacking and have seen people have similar problems with blisters between the toes. While this is sometimes caused by boots that are to tight, some peoples toes have a tendency to rub do to the shape of the toes. The extra pressure on the foot due to a pack or in your case fins causes the blisters to form. Try wrapping surgical tape around each toe involved. Make sure it's not to tight. For backpacking I sometimes use a product called "mole skin" or "mole foam". It's an adhesive backed bandage for cushioning blisters. The "foam" is thicker then the "skin". I haven't used either product in the water, so I don't know how well it will adhere.
One other thought, again based on backpacking experience, make sure your toe nails are cut.
One other thought, again based on backpacking experience, make sure your toe nails are cut.