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.