Recurring (every couple of years) blister below my shoulder blade. Painful, inflamed & always occurs in exact same spot. I've never had chicken pox.

Could be herpes. See a dermatologist, best to go immediatly (same day) when you get the next one.
Hard to say. Without seeing the blisters. It is best to have it checked out and get treated . Also if you are absolutely positive that you have not had chickenpox at all, would highly recommend getting the vaccine asap. Please talk to your pcp about it.