Robbie, Big-Penis.com’s forums are now at penis.vc, although it looks like a lot of posts were lost during the domain changeover and I wouldn’t expect a lot of activity there anymore. You should contact Julian, who posts there as Phalomed about your blister issue. I know he’s posted his email address there but I couldn’t find it right away.
I seem to remember people over there having the same issue a year or two ago and the answer was either to use a slight amount of lube before putting it on, or that you could be an in between size, where for example you would need a medium bell on a large condom or vice versa. I would get in touch with Julian or somebody at Phallosan. I don’t remember reading anything bad about their customer service.