I put blue (very thin) tape directly on my thumb. Takes some getting used to, but I put it on the palm side of my thumb, most people look at me as if the ball show fly behind me or something. But many people put it on the back of their fingers and/or thumb.
https://www.amazon.com/Vise-Hada-Patch-Pack-1/dp/B003ZCD20A
Please sponsor your local Pro Shop. I love amazon, but only linked it for informational purposes.
I use blue thumb tape after running into similar frustrations. After a few iterations of thumbholes really reduce the irritation I still use the tape for comfort.
So, I would suggest using the thumb tape to prevent the blisters; then as you get equipment mention to your PSO the thumb troubles you've had and see if some minor pitch or span adjustments can fix it.
You still may need to sand the hole down with some emery cloth, but you will notice a big difference when you keep the thumb flat. You are not going to drop the ball if the thumb is flat (I thought I was going to, but it will not happen unless the thumb hole is too big). Also, buy some Vise Hada Patch Pack #1 blue tape for your thumb. Put it over the part that the blister is on once the blister heals. It makes a huge difference! Amazon has it cheap. That is where I got it. http://www.amazon.com/gp/product/B003ZCD20A?psc=1&redirect=true&ref_=oh_aui_detailpage_o09_s00
I actually use Vise Hada precut thumb tape. The red version works best for me. I put it on my thumb where I get the most friction...
Thumb tape to snug up the hole will be very important also to totally eliminate your grip pressure. You put the tape in the thumb hole to change the size
Something like this: https://www.amazon.com/Vise-Hada-Patch-Pack-1/dp/B003ZCD20A/ref=sr_1_1?ie=UTF8&qid=1493218627&sr=8-1
Test your thumb fit with this video: https://www.youtube.com/watch?v=wdmcS2luEf0&index=10&list=PLuxkgyCIqmnoAR6dYyZHZtTKkw5WMb4J-
This for the back of your thumb. http://www.amazon.com/Vise-Hada-Patch-Pack-1/dp/B003ZCD20A
This for your fingers. http://www.buddiesproshop.com/p-4379-vise-bio-skin-pro-tape-silver-roll.aspx