I agree that it is probably more a mental exercise, but if you are not comfortable with it, definitely don't do it, especially if you are concerned about how it will affect your stress / blood pressure level. I have to say if it were me, I probably wouldn't be comfortable with it either, but I can see how it would be beneficial to those who want to prepare for the experience of labor and working through the pain.
Labor is different for each woman, but I will tell you that it does hurt. A lot. I'm not trying to scare you, but I don't think it is a good idea to go into labor thinking it may not hurt, when the reality is that it does.
Even with hypnobirthing, it is a matter of training your mind to work in a way to combat the neuro-input from the natural pain your body is feeling by putting a focus on comfort. I'm not saying it won't work - I'm sure it does to some extent, and I hope the best for you. I just don't think personally that anyone can be pain free completely during labor unless you have something like an epidural, and even then the labor before the epidural is difficult and painful. Please note that I am not saying an epidural is *better* than hypnobirthing at ALL and I hope you have a great expereince with hypnobirthing!! :)
The thing that helped a lot for me was remembering that even though it hurt, it was a GOOD thing, and that this pain wasn't a signal that something bad was happening. It was my body working to do what it naturally does - it just happens to be uncomfortable. Every time I would have a difficult contraction, I would squeeze something hard (usually hubby's arm - bless him) and put my focus on that sensation to help distract the nerves from sending pain signals from my abdomen. I was still in a lot of pain, but it made it more tolerable.
It DOES help the pain level not to panic about it, which I think is the core of why hypnobirthing works on some level. It helps to keep you calm and work through the pain to a place of serenity in spite of it. I will say though, that the the founders of HypnoBirthing and Hypnobabies both don't promise a pain free birth. I hope the best for you, but I know with my labors & births the pain was MUCH more intense than holding my arms out for a long period of time.