This is a really great foot care book, with a lot of info on blisters. http://www.fixingyourfeet.com/

The author will also personally offer advice for your particular situation- he's very available.

Also, here is an article on the relationship of blisters/black toenails to electrolyte status. http://succeedscaps.com/blisters.html

I have run the same trail marathon three years in a row, getting nice blisters and losing toenails every year until this last, where instead of trying to watch the clock and take electrolytes I actually set a timer and took them at 30 minute intervals without missing any. I didn't get any blisters- wearing same shoes, same socks, same foot prep, same weather conditions, same course. I'm a believer.

Nanci