This sounds like a friction blister, not herpes. I'm not sure what test the doctor wants to do. I suppose culturing a blister when it's fresh is easy enough, but it's not likely to be useful.
It seems to me that your own analysis is correct, and that you should do best by minimizing friction any way you can, as you have been doing. Lubrication is the key.
If you can't prevent these blisters by lubrication, you should show them to a dermatologist or urologist when they're fresh (or photograph them).
Best.
Dr. Rockoff