Hi Bob,
I have recently started making these blanks and I recognised that blister right away as I had the same thing happen to a couple that I made. I found that I had to be very thorough about sealing the paper before I cast it in resin. Check out this post:
http://www.penturners.co.uk/casting/12836-two-firsts-once.html
The slightest bit of unsealed paper will leak air as the resin sets, giving exactly the problem you are seeing. I would say that the blank is faulty, and that nothing you have done would cause this. If you had looked very carefully at the blank before you turned it, you would probably have seen this, although it can be easy to miss in the raw state.